Just wondering (this might have been asked before) how much skill on the skill bar will a villager gain in one year, and/or what does it depend on?

There's really no answer to that question. It depends upon what the skill is, how much walking around the villager does, how much skill a villager already has, what the villager's likes and dislikes are, as well as their innate abilities. Every villager and every skill is different.

Yeah, I get it, but say when the game is off, and I'm playing on 2x speed. I come back in one hour, what type would have gained the most skill. Farmer, Builder, Scientist, etc.? or is there no answer.

if you had a runner, you know they would !!!