A few days! Heck, it should be dealt with prompter than that. When a shop closes, and Shopify sends the App Uninstalled webhook, we’re talking mere minutes at the most! Why wait around days! It is likely that recycling those names is possible, but I would not stress on it either, as you always start with a fresh slate of name, and ID’s if you’re doing it right.
I think I still own flowers.myshopify.com and cars.myshopify.com if anybody wants… haha. Going back to 2006 I bet. When it failed to occur to me that shop domain name matters for nothing save it is the unique ID of the shop.