How long to wait so pet is not "full" anymore? Is there anything I can do to make it hungry again? TIA

It used to be that your pet had a chance at losing one hunger every hour or so. I don't know if it still works like that, but hunger was glitched recently to drop a lot slower so it may still be (in all honesty I don't keep up with it that much)

The only way to make your pets hungry is to use in battle with a different pet, but it's tier 6 and only has a chance of activating that effect in battle. It's not worth it in my opinion, but if you have the money or someone to borrow from, and a tier 6+ pet that's not also your gourmand, it's an option.

Thanks for the insight! Have a great weekend!