Once you are asked to leave how long does it take before your allowed to go back to check for items?

I believe it's random, up to an hour or more. I'm not 100% sure so maybe someone else will have more information. It's best to not check more than once every 20 minutes or so to not get kicked out, and don't linger on the page if you see the shop is empty.

it's been as little as twenty minutes for me, but one time it was several hours. I don't think it counts against you has "having visited too often" if you are banned and just checking in to see if you've been unbanned yet, though.

and Thank you both. :)