Shops only reset when you level up. There is no reset time. Here's a response from GGG:
Here is a link to the post.Shops only reset on character level, this is intentional and has been the case for a long time.
One of the reasons this is done is to prevent situations in which people can level up multiple characters to a certain level and ultimately be able to farm valuable items (5-link, 6-link, 6-socket) with zero effort.