When can I go on another pickpocketing spree?

Solution 1:

As far as I can determine, for named NPCs; their inventory does not respawn (additional source).

So if you pickpocket, say, Jarl Balgruuf; his inventory is static from when he is first loaded, and anything you pickpocket off him will never return. (Insert region here) Guards, however, are not necessarily static, and may have their inventories reset with the rest of the map cell they reside in if it has not been visited in 10 in-game days.

There may be some odd exception cases; particularly if a specific NPC is called upon by certain quests, it may generate a fresh version of that NPC complete with fresh inventory.