How often do shops restock?

Solution 1:

You don't have to wait at all if you aren't currently engaged in a co-op game. Just log out and log in. When the world is re-created, all of the vendors will restock!

(N.B., this will regenerate all random areas, destroying any loot you might've left on the ground on the other side of your town portal (along with the portal itself), so make sure you've cleaned up and are ready to hop a waypoint and reeler to wherever you left off.)

Solution 2:

Exiting the game you are in and creating a new one will refresh the shop's inventory. This is a quicker way of looking for a specific item from shops.

I'll try to test how long in-game time it takes for shops to refresh when I can log back in.

Hope this helps a little in the mean time :).