The incubation period is dependent on the virus which causes the infection. Generally the incubation period can vary from 4 hours to 3 days. In case the infection is due to adenovirus then, the incubation period can even be a week
Normally, 1 to 3 days is the incubation period for stomach flu symptoms. However, if the symptoms get severe, seek medical advice from a doctor. At this time, symptoms such as fever, nausea, vomiting and diarrhea can be seen.
Generally it takes 4 hours to a couple of days for the symptoms to show up after the infection. In case of astrovirus, this duration can even be 1 week. Hence this depends on the virus that has caused the problem
To ease your upset stomach you can take antivirals, ginger ale and ginger tea. Vitamin C also works good for this purpose. The symptoms occur only after the incubation of the virus inside your body(5-48 hours).