Generally fasting under 2 weeks is considered safe. The stomach does not have a problem being in an acidic environment, it's always in acid. What some people report is heartburn or acid reflux, which can be damaging to the throat lining however it's usually temporary or treatable. Keep in mind people have been fasting safely, whether intentionally or not since before history. Our bodies are capable of handling many feast/famine cycles.

Typical advice I've heard is to not stay hungry for too long. The stomach sends acid to the stomach as a strong message that it's hungry. Not eating means there isn't food protecting the stomach from the acid. That risks getting an ulcer.

Fasting seems to be a common practice, though. I wonder what variations in fasting (how long to go without eating, how often to fast, etc.) maximizes fasting time while keeping the risk of getting an ulcer to a minimum.

EDIT: Changed "increases or decreases risk" as of course fasting is not going to decrease the risk as opposed to not fasting.