On the upload screen if you used all your upload slots it says "You have uploaded as many images as you are allowed to for today." It'd be kinda nice if it said how much time you need to wait till you can upload another...

Basically, you can deduce that yourself. Search for the image you uploaded first. Look at the time. Now expect your upload slots to be back again at two, three, four hours from your last upload. Sometimes even six. So I kind of "feel" the time my first upload slot is back.