Drank a chocolate milkshake, when can I sleep?

I wouldn't worry about it. While eating too close to going to bed is not a great idea for several reasons, a milkshake (unless huge.....) won't present much of a problem. It would be a good idea to brush your teeth well and use mouth wash before going to bed however.. otherwise you'll wake up with fuzz growing on your teeth (i hate it when that happens :P)