Sliding Scale of Turn Realism

Games that simulate real life to a degree have, over the years, had to deal with one problem time and time again. How do you simulate the passage of time in a situation where tracking it is vital? While you can say "about an hour" when dealing with shopping, in combat you need exact measurements. There have been many different attempts at this.

There are three examples used here, blowing smoke from a smoking barrel (less than a second), loading a handgun (4 seconds) and field stripping a gun (one minute).

Second by Second

Each second of combat time is described out as an action. Actions taking longer than a second are broken down into second long chunks so in effect everyone acts on every turn. This is most realistic but also rather book-keeping intensive. Blowing the smoke from the barrel takes 1 second. Loading a handgun is broken down into 4 equal-length actions such as unclipping the magazine, removing the magazine, putting a new magazine in and re-engaging the mechanism. Field stripping a rifle would take 60 such actions and might just be handwaved as taking that time, rather than broken down.

GURPS generally uses one second combat round, but see below.
Hackmaster (at least, the newer editions) measures combat second by second. You can move every second, but the amount of time between 'significant' actions (attacks, spellcasting, and the like) is semi-random. The time between actions represents preparing, re-readying weapons, and the like.
Most computer games that are not turn-based still calculate everything by the second or part second, making them very very fast real time turn based games. (Many games use imperceptibly short turns; some shooters process 30 to 100 'turns' per second.)

Other incremental

Sword’s Path Glory by Leading Edge Games was a tabletop game had turns that lasted 1/12 of a second.
AD&D1 used 6-second "segments" - which is why it introduced weapon Speed Factor, that became a nonsensical atavism in uniform rounds. Speaking of atavism, it also measured time in rounds and turns in addition to minutes and hours - this probably was the reason why the segment system was scrapped when AD&D1 mess was cleaned up for AD&D2.
Alternity 2017 has round split into 8 "impulses", with different length of actions and weapon speeds - AD&D1 style initiative, but with streamlined rules.

Action by Action

A choice used extensively by White Wolf, similar to Second by Second in that time is broken down into seconds but different in that if an action takes longer than a second then it occurs instantaneously and then the character simply does not act for several seconds worth of time. Less realistic (as you have sword blows hitting and then the person spending time on the attack, for example) but easier to administer. Blowing away the smoke from the barrel still takes 1 second, allowing you to act next second. Loading a handgun takes 4 seconds, meaning that you load it on second 1 and then do nothing for seconds 2-4. Stripping the rifle still takes 60 seconds and again the detail would probably be ignored.

Champions (A segment is one second, 12 segments is a round. A character's SPEED stat is how many times a round they can act, the default speed is 2.)
Exalted
Fallout (by virtue of having a limited number of action points per turn)
The Active Time Battle in the Final Fantasy games
Scion
Most Roguelikes
GURPS developers eventually realized that a metronome has a few problems when trying to emulate action-movie realism with rubbery time, so Action supplement uses Turn by Turn, with the length of a turn defined as "the time needed to do something cool". It even includes a rule that if a bomb was ticking, then the remaining time is reduced by a random amount, regardless of the chase scene length for this!

Round by Round

Time is sliced up into chunks and in each chunk everyone acts. The length of the round will vary a lot by game, ranging from a few seconds to months or even years. So long as they have a defined length, it is Round by Round. The method used by Dungeons & Dragons, as well as most RPGs, this is less realistic but easier to administer again. Some actions have to span multiple rounds however and other actions seems to take a long time compared to what it would take in real life, for example pushing a button could take as much as 6 seconds if that is the length of a round. This can often also lead to problems transitioning non-combat actions into combat situations due to the duration differences. If a round is 6 seconds then blowing away the smoke would either take the whole 6 seconds or be ignored time wise. Loading a handgun would take 2 seconds longer than under other systems. Stripping the rifle takes 10 successive rounds. Some systems break the round down into portions to allow some flexibility.

Civilization - In the beginning, each round simulates up to 100 years, but the amount of time spent during each round lessens as the game continues, presumably to keep it realistic. It would be crazy if it took the same amount of time for a stone age civilization to create a barracks as a modern one.
Diplomacy - 6 months a round
Dungeons & Dragons - all except AD&D1. 3rd edition and later (and D20 system games in general) - 6 seconds a combat Round subdivided into specific action types in newer editions. From the Original D&D to AD&D2 a combat round is 1 minute and a noncombat turn 10 minutes. In Player's Option ("AD&D 2.5") combat round is 12 seconds, 50 rounds/turn.
Gamma World - Route moves (about 4 hours) and either search moves (10 seconds) or combat melee rounds (also 10 seconds), depending on the PC's actions.
Sid Meier's Alpha Centauri - 1 year a Round
The Total War series (for the turn-based part of the game, anyway). The amount of time per turn varies from game to game; Rome: Total War for example has every turn represent 6 months, while Medieval 2 Total War has turns representing two whole years.

Turn by Turn

Used by most boardgames and Paranoia, this is essentially the same as Round by Round, save that each round has no specific duration attached to it. Everyone gets to act, and then everyone gets to act again. The least realistic it is also the easiest to do the administration of. Blowing away the smoke would almost certainly be ignored time wise. Loading a gun would either be ignored or take the same, abstract, amount of time as stripping a rifle.

Most Boardgames and RPGs
Paranoia
Pokémon. However, some attention is paid to moves of unusual duration. Particularly fast moves always go first, slower ones always go last, and some especially slow ones waste a turn charging up or recovering.
Final Fantasy at least in the early games and X before they went to Active Time Battles.
Warhammer Fantasy Battle and Warhammer 40 K
Warmachine and Hordes.
Interactive Fiction games.
Some comic book (but not only) RPGs have turns based on 'panels' - that is, each hero has one panel in which he can make any action normally able to be depicted in a comic book panel. Seventh Sea bases this on your charisma - the more handsome and cooler the hero is, the more often the 'camera' focuses on him.
Knights of the Dinner Table has Brian and Weird Pete as the last participants in a long running war game based on World War 1. They meet once a month with each one carrying out a single turn which takes hours to complete. Though almost certain, the two players involved are a factor.
The World of Darkness

Under these systems there is also a distinction between prioritised and non-prioritised. In prioritised the involved parties all act one after another, each finishing everything they are doing before the next person does anything. In non-prioritised (Simultaneous Turn Resolution) every person's actions happen at the exact same time and overlap.

The Paranoia supplement The Bot Abusers' Manual has an interesting mechanic for how long a bot's power supply lasts between charges. The theory is that a short period of game time doing something interesting (twenty seconds shooting at Commies) should burn as much power as a long period of game time spent doing something routine (two hours wandering down corridors) - but both of these take about the same amount of real-world time to play out, so the GM just winds up a kitchen timer and lets it run down at a constant rate.