You don't need to kill a mob before it can be pickpocket'ed again, they're pockets are on about an 7 minute respawn timer.
Okay, I know there's a lockpicking sticky at the top of the page, but I read the whole thread and didn't see this anywhere. I made a new thread because I don't think Rogue vets (such as myself) will bother reading every post.
Anyway, I got lazy with my lockpicking and hit 70 with only 134 skill. So I decided to powerlevel it fast and got to 250 when I went to Tyr's hand.
I found it tedious having to kill all the mobs (I'm not too concerned about the drops, just want the skill-ups) so I tested out a theory. I pickpocketed a mob and stood near it (to make sure no one came along and killed it) and checked its pockets every now and again. After about 7 minutes its pockets were refilled.
This was hella useful because all I had to do was start at the entrance and work my way to the end, then start over without having to waste time killing everything. It took enough time that the pockets had respawned by the time I started over.
If everyone else knows this, fair enough but I sure didn't, so, hope it helps.