As for Main campaign, it can be as short as 10 minutes short (speed runners player) to many, many hours (normal players). Its usually long if ya take ya time. Noted though, in Fallout 1, ya given a time limit in a certain amount of days to finish the first part of the main quest. Afterward, ya given a reasonable amount of days to finish that game it self. In Fallout 2, many, many years before the game would hit "game over" if ya procrastinate the Main Quest that long.
There're a lot of video's on speedruns so it can be done. However I tried it once in Fallout 2 and to be honest the game kicked my butt, enclave troopers just sliced me in half and had me for breakfast. :laugh:
You'd probably have to use a lot of saves and have a lot of luck.
-------------------
On topic,
Some people hate the classics and some love it. If you don't expect it to be like FO3 there is a chance you like it. It's a turn based isometric view game.
To be honest FONV has a lot more in common with the classics than FO3.
In my opinion the story, dialog, karma/reputation, character system, humor, towns, NPC's, quests, feeling of accomplishment are a lot better.
There is also a lot to explore in FO1&2, only different, it's not like walking through a huge world because the worldmap is based on lets say point & click mechanics but you'll get to explore settlements and special encounters.
The main campaign is relatively short but if you don't know what you're doing it takes a while to uncover it because you'll have to visit a lot of towns to do so and there are also a fair amount of side quests.
Game time, well it depends like I said if you're new to this and don't read any spoilers it can mount up to weeks of fun. Even for a veteran (no speedruns) it will take 1/2 day up to a few days at least. After the end you can still carry on if you want to.
Difference between game size of FO1 and FO2 is that FO2 is double in size.
Some people find Fallout 2 a bit goofier with all the pop culture references, although I like FO2 because of its size. But it's true FO1 was more serious in that respect and had a darker sense of humor.