If exposed to the stomach virus when can i expect to start showing symptoms?

Gastroenteritis: The most common "stomach virus" (causing viral gastroenteritis) is norovirus. Symptoms usually start 1-3 days after exposure.

2-4 days, but...: It varies among different "stomach viruses" and among different people, so is not predictable. However, one can feel safe if she wakes up on Day 5 without any symptoms. If unlucky, she can still come down with the illness up to about Day 7 or 10. That means she is considered completely safe if she wakes up on Day 11 without any symptoms. Most people who get sick likely have symptoms on Day 2 or 3.