This isn't the one that has a time limit, is it? IIRC Fallout 1 or 2 had a certain amount of time or in-game days or something to complete the main task
both have time limits, sort of.
the first main quest has a hundred-something day limit, how many days left are shown on a sticky note on your pip-boy. if you just stick around for a long time during the two last main quests you'll find some towns after time will *change* leaving you with a different ending and you cant do anymore quests in those particular towns.
fallout 2 has a very long "hidden" time limit on the main quest. when the time runs out your village dies.