GTA Online doesn't make it obvious how long an in-game day is, and some players won't bother counting or calculating it. Just know that 48 real-life minutes is equal to one day in the game, which is the same for other HD Universe titles like Grand Theft Auto 5.
GTA 5 and GTA Online have a persistent in-game time passage system. Two seconds in the real world equal one GTA 5 minute. Time can be checked from the Pause Menu or from a player's Cell Phone.