Stat Questions

What I know: AA based champs build AD. Ability based champs build AP. Anyone with bad scaling builds the relevent penetration stat.

What i don't know: What's a good amount of scaling for stats like AD, AP, Armor, and MR? How much CC (Slow duration, slow amount, stun duration, root duration, etc.) is good? Do all stats grow as you level? Do i get MS per level? How much Life steal is worth going for, specifically on an AP assassin with great AP scaling? Is going for a large amount of MS good and will it ever be useful?