How Long Until Dog Passes Aluminum?

Two days ago, my cocker spaniel ate most of a small aluminum bread pan--the disposable kind you can buy in the supermarket. The pieces I picked up when I discovered what he'd done were sharp, so I called the vet (right before closing), who recommended just keeping an eye on him for nausea, diarrhea, etc. The next morning I brought the dog and he showed no discomfort, but the vet put him on an antibiotic (esp. for gastric problems) just in case. It's been 48 hours, and the dog's still happy and eating and drinking, and when we go for a walk, his poo is fine (yep; I examine it!) ... if he's passed the aluminum, it must have been in the back yard when he was out on his own, and not on a walk.

However, I'm still very anxious. At what point can I draw a sigh of relief? Thank you!