The in game clock.

[I Shot The Sheriff]

24 seconds per day.

[Andreas]

Well, I think it should be at least 48 minutes or more like 72 minutes. Both would be definitely better than the GTA III era where a real life day were 24 ingame minutes which is way too short because you can see how the colour of the sky changed and that within seconds, and also the shadows in San Andreas moved pretty fast. GTA IV was an improvement and it wouldn't be bad if Rockstar extend the day once again in GTA V considering that the map is going to be really huge.

[.dre.]

I want them to push it to 48 and see how it works out.