Empty stomach?

Kind of a simple question, here: certain supplements and such things are intended to be taken “on an empty stomach”, particularly amino acids (as they have to compete with those in the food to get into various parts of the body). That said, how long after eating, say, a typical meal has your stomach more-or-less emptied itself?

Obviously, there are endless variables, here, but I’m looking for a rough estimate. Half an hour? An hour? Several hours?

Thanks for any input.

-occ