That’s really weird. Educate them so they understand tan changes the colour of skin and that it’s not makeup.
I can only assume these that don’t want them to last are having them way too dark or for shows or competitions, or nightclub appropriate only and want it off so they don’t look silly on mondays at work. Or they are just barmy and like wasting money. In that case, if you cannot convince them to have a more natural shade, with how to keep it looking good ongoing, tell them not to prepare with exfoliation so they are sprayed onto dead skin and retail them a removal mitt or product or both and it will be off very quickly. But also cover yourself so they know that will not look good if they don’t remove it properly.