... Man, Fallout 1 was one of the few games I bothered finishing. Being really short helped.
How long is Fallout 1? I started it a couple of months ago and I haven't gotten back to it since. I think it's better than Fallout 3 in terms of how much variety there was in combat in how more strategy is involved in it.
Depends on how much you know about what you're doing. If I remember right you can speed run it in about 12-minutes. If you wanna run around and tag most of the towns and do about maybe half the questing I'd say you could get it done in roughly 4-6 hours. If you have nooooo idea where you're going, how to build, ect. Could easily get 2-3 solid days of nonstop playing counting repeats.
Then there's the replay factor. Different builds, being good/bad, the non-canon alternate ending, ect.