To answer your questions, I've been doing game dev for a little over a year now and how long it takes depends entirely on what skills you have and how much effort you want to put in. If you're an excellent programmer you could focus on making a very fun mechanic that feels good and just give the art to someone else and then bam you've made a good game. Or you could learn art yourself and that would take much longer.
As for learning art, I only know about 2D art really but the way I learned it is through repetition and through copying other people's styles. If you think that a particular piece of game art looks good you should look at it and try and determine why you think it looks good and what specifically is making it look good. Then try and think about how they went about it, then try and do something similar when making your own art. Personally I think its a good idea to start with pixel art, because when something looks off its super obvious what it is. For example if something is supposed to be circular, its immediately obvious why it doesnt look right because there is a correct way to draw a circle using pixels for every possible diameter. However, some people think sprite based art is better because its harder to notice mistakes.
Hope this information is helpful!