Long time, that's what basically pisses me off about this game above all else. Not only does it take a long time you have to level up everything and can't use half of everything.

Few tips.

Alteration, use telekinesis to hold up an object levels fast
Smith, go town to town buy leather and iron make daggers and gloves
Conjuration, summon deadra, hit it four times at least then finish it and keep resummoning
Illusion, spam courage
Restoration, just find some trap such as the moving blades and keep healing yourself
Destruction, rune traps seem to give good amounts of level
Archery, if you got Fendel early on you can have him level it up 5 times and get your gold back get to level 50 archery easy
Joining the Companions allows you to get 5 extra levels per level without paying anything
Sneak, back stab gray beards, they don't die or attack unless it's four or more
Speech will take a long time, better off just going to bards and train it but unable to get gold back

Even power leveling takes a long time. My character is around 50