According to my brother one week is roughly 8 hours of game play (if you don't sleep or ride anywhere), and suggested that you give people 8 real hours to play and 24 hours to hold onto the save. They end at whichever comes sooner, playing for 8 hours or having held the save for a day. Or getting bored.
In my opinion, you should just tell people they have to keep some kind of journal of their adventure (lots of screenshots and a caption for each). That would make them play a bit slower.
But I really think 3-4 hours is probably more reasonable. 3 people playing 8 hours could probably finish the game, haha.