When shooting content, when specifically do you feel the 'ideal' time for the models to get totally naked would be?

For example, lets say you are working a 20 minute scene with two models, how much of that scene should show the models still wearing clothing, at what point should the clothing at least start coming off and, at what stage should the models be 100% totally naked?

Do you feel that leaving a models clothing on can actually increase the effectiveness of a shoot if it is timed correctly so that they remove they clothing 'naturally' as oppose to literally stripping everything off at the first chance possible?

Your thoughts please?

Regards,

Lee