Shops restock at random minutes on the minute. Well actually it depends...it can be anywhere from the :00 secon to the :20 second mark depending on the shop number. You can tell the shop numbe by the shop's URL:
http://www.neopets.com/objects.phtml?ty ... obj_type=1
Food shop is number 1
http://www.neopets.com/objects.phtml?ty ... bj_type=48
Usukiland is number 48.
The bigger the shop number, the later it restocks. What minute the shop restocks on is random. A good place to restock depends, places like Snow Food and The Bakery are good and have lots of good items but are very crowded. Places like Pizzaroo have fewer good items but are less crowded. I suggest choosing a shop and just trying it out for a few days. You will probably loose some money, but you will also figure out what items are good, and which are bad. A good gudie to restocking is:
http://home.neopets.com/templates/homep ... tockopedia
It is very, very helpful.
Well I hope I helped, and made sense