I said fifteen minutes, but the fact of the matter is that I would never change a diaper in public.
My mother once showed me how it was possible to change the diaper while RoseyCheeks was still in the stroller (about five months old). She did just fine and no one noticed that she was changing a diaper . . . until, of course, she had to take the soiled diaper and throw it away.
Of course, the first time I ever stood up to my MIL was about changing a diaper in public. RoseyCheeks was four months old, and we were at an outdoor mall. RC needed her diaper changed, but this shopping venture was at the tail end of ten days of "We do X" and "We do Y." We couldn't find a restroom very easily, so MIL said to RC, "It's okay. We're not too proud to change you on the bench, are we?" And I jumped in with, "No, wait! Sorry, but we are too proud!" I was as polite as I could be, and it really only took us about ten minutes to find a bathroom (although we did have to walk half a mile), but I have a thing about changing diapers in public.