4 hours is typical of the groups I've been in, most typically from 6 to 10 or 5 to 9.
I've had shorter ones, like 2.5 hours for a Star Wars campaign when it was being run after our campus gaming club meetings which typically ended around 7:30 and the building closed at 10:00.
I've also had much longer ones. My first D&D campaign had 8 or 9 hours sessions regularly: 4 PM saturday to midnight or 1 AM Sunday morning. Last year for several months I was in a nWoD campaign that was supposed to have 8 hour sessions (2 PM to 10 PM on Sunday), but was more in practice 10 or 11 hour sessions (1 PM, she'd start running as soon as most people were there, even if you arrived "on time" before 2 the game might be going if she wanted to start earlier, and she'd run until she felt she was done for the night, which often meant going until midnight, which was hard on those of us who had to work the next day.)