How does sneaking work (i aways play a rogue type character)?
AFAIK, the sneaking skill only reduces the noise you make when walking (or running if you take the Silent Running perk). Once hostiles have Line of Sight to you then combat starts and they gain initiative. So you have to be careful to only sneak when their backs are turned.
Is it possible to snipe things from afar ala jagged alliance?
Enemies only see you when at a certain range and you can start combat (turn based) mode at any time, so yes, you can fire with a long range weapon. There are sniper rifles and later there's gauss rifles with have huge range. Laser rifles and hunting rifles aren't too shabby either. There's no separate "snipe" action.
Is it nescessary to have an actual party, or i can choose to play with only my little vault dweller?
Your followers have their own AI and you have no control, so most of the time they'll simply draw a bit of attention away from you, and will shoot at targets of opportunity, but you can't really micromanage them into an engine of fiery death like in Baldur's Gate, so yeah, I'd say they're optional: they help a little but they're not super useful. And since death is permanent, more often you'll find yourself reloading a game because one of the followers was stupid to run right in the middle of 50 super mutants and got flayed, killed, raped and incinerated, in that order. Also, sometimes they're assholes and spray you with bullets to kill the giant foot-long radroach that was behind you, even when asked to avoid friendly fire.
How long is the game, in hours, playing on normal difficulty and not wanting to get every single thing in the game (which i will, if i like it)?
Fallout 1 and 2 are the only games where I've only done whatever "side quests" I felt like doing, because I found about them, and not try to solve every single thing for completeness sake. It feels very organic... no need to force yourself to take every single side mission. It is a good idea 'tho to visit every locale even if you don't do every "quest".
Does my character's animation actually changes when he's sneaking (sort of a random question, but i hated how my character on baldurs gate just went semi transparent and stood there with the posture of a tango dancer while supposedly sneaking)?
No, I don't think there's any kind of visual feedback when you're sneaking. But I think a little green text that says "sneaking" pops up somewhere and it disappears if you do anything that cancels stealth mode.