I don't know about PC, because it depends on your hardware configuration. In other words - how fast your computer runs.
But on my PS4 it took about 5 hours. The main reason of long installation - your internet connection speed. At first point you receive a small installer of GTA 5, which loads the rest of the game from Rockstar servers (about 60GB). So you can approximately count the time required to download and then unpack such a huge amount of data.
But, ofcourse, it is worth waiting!