If I recall correctly, ingredients were on a 30 day respawn timer in Oblivion. I assume the same of Skyrim. As for leveling Alchemy. Remember to activate the Thief Stone before any potion-making sessions. As for eating the ingredients, don't bother. The amount of experience is minimal compared to the amount you would receive from making pots from those same ingredients. Of course, better pots equals better experience. You can google good recipes. There are some 5-effect potions that aren't hard to make, yet yield a good deal of exp and are worth a boat-load of gold. Good profit.
Additionally, for the lower levels, use Arcadia (spelling?), the Alchemy Shopkeeper in Whiterun. She can train you 5 levels of Alchemy per Level Up you get. You can then pickpocket your gold back and repeat (be sure to save first). Unfortunately, you'll likely have to pickpocket between every training "session" (level purchased), as it is nearly impossible to pickpocket 2000+ gold haha.
It's a lengthy process, but the results are pretty amazing. Happy grinding