The box says ten to twenty minutes. Almost nobody follows it. At twenty minutes the sheet is still wet, throwing away something that still has liquid in it feels wasteful, so it stays on through one more episode and comes off stiff and papery somewhere past forty. The reasoning underneath that is fair enough. Liquid left in the sheet looks like work left to do, and a dry sheet looks like proof you got every drop. Sheet masks are sold as the treat step of a routine, so longer feels like more treat. What changes over...