it should restock when the inventory restocks which is every 48 hours.
however skyrim is very strange when it comes to cell buffer managment.
if you quicksave, kill a merchant, the quickload, his inventory and gold will be reset.
if you quicksave, buy a merchant's items, then reload, their stock will not have reverted back to what it should be.
This reminds me of an extremly weird bug in Fallout 3, wanted to enslave a lady in tenpeny tower, saved and used the stun gun on here, however she run away then I tried to collar her so I reloaded.
Now her was gone, quest marker pointed to the slave camp for some reason, but quest said she was not enslaved so I fast traveled up, just to find that she was walking home to the tower, new fast travel and I intercepted her and collared her.