Hello,
Yesterday our German Shepherd ate half of a hand towel before we noticed her doing it.
The vet told us to smash her poop to look for towel remnants, but so far nothing. How long should we wait before getting worried? We forgot to ask the vet that.
Thanks for your help.

Phone the vet and ask now. The towel pieces can become entangled in intestines, necessitating a very expensive bowel operation by the vet.

Thanks, we'll do that.
We weren't sure if we were being overly concerned. Guess not.

No you aren't being overly concerned at all! Yes please take her into the vet asap! I just watched a show lastnite on the Animal Channel where a dog had eaten an area rug. The dog seemed to be fine, but they took him in. He was bleeding internally, and it had ruptured his intestines, and esophagus! Thankfully they were able to save him, but it was critical, and the dog was lucky to survive. He now needs to be on medication, probably for the rest of his precious life, and the vet bill was horrendous, because they waited. Please take your dog to the vet!