Your shop window is the first thing customers see when they walk past your shop.
The window is there to entice customers to come in by giving them a sample of what you have to offer.
You need to change it as often as you expect regular customers to visit you.This is not an issue with people who do not know about you but you need to make the effort for your usual customers.
For instance, if your regular customers come to you once a month, you need to refresh the display in the window once every three to four weeks.
If they visit you once a week, make sure the window is updated twice weekly.
This ensures customers always see something new and different when they visit you.