The same as in DD, as you do quests and explore new areas. I never did figure out the exact times that they restock. I think it is mostly related to quests you completed. Or major finds such as a new cave or recently killed boss.
If anyone knows how it worked in DD (I randomly checked) then it is how it works here too.
Last edited by LightningLockey; 13/12/09 06:57 PM.