It all depends on how you approach the games. If you want to try a sneaky diplomat it might take a little bit longer because there might be some combat situations that you might have to reload a few times to get through them
When I played the Fallouts when they first came out, it took me about 1-2 weeks to beat each one and that was with just playing Fallout 1/2 and nothing else. School had a way of eating into my Fallout sessions XD.
Besides, there are tons of side quests to do in each game, more so in Fallout 2, and those should keep you occupied for a while.