For dental caries, 6-10 grams per day [1]. Most adults can tolerate 40g.

Diarrhea is a concern at 45g for children and 100g for adults. If you introduce it gradually into your diet, it should not be a problem.

1. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4232036/

Health concerns for humans is potential digestive issues at high amounts (many tends of grams). Health concerns for canines is prolonged excruciating death due to live failure in rather small amounts. Don't let your dogs near xylitol.