Fallout 1 can take about 20 hours on the first play-through, and less on Fallout 2 if you played 1, but god forbid you install the Restoration Project. So much new content.
As far as Linearity, TES is more "Open" but Fallout itself is far less linear. Pretty much the beginning and, to a minimal degree, the end are the same every play through. But the journey really ends up being an odd, Random play-though each time.
The Fallout games are based on Statistics and virtual dice rolls, though they happen behind the scenes so you never have to worry about it. Aside from that, random luck, what you say, and where you stumble can all vastly change the course of the entire game, and do it more profoundly then simply skipping a massive section of the game like TES games and Fallout 3.
In one Fallout play-through I completely broke the order of play and ended up near one of the final areas very early on in a completely different kind of situation then I had ever been in before. I managed to get out of their using a very James Bond epic sequence of events involving trickery and sheer luck, and the rest of the game (which still was there to be played, just because I did a final area didn't mean I'd beaten it yet) played completely differently then it had ever done before.

To sum it up, Fallout truly lets you write your own story, and quests don't really have a set start and end point like the TES games are cursed with. The experience is much more fluid.