In the same way one can do it by daytime - taking a guess, and rough approximation By remembering what you did during a LD, it should be feasible to give an estimate of how much time was required to do it. If you are unsure about the duration of your dream, take the middle value. If you think your dream lasted 3-5 minutes, it should be safe to take the 4 minutes mark; if you are thinking 2-4, however, it would be best to assume it lasted 3 minutes and take the 1 minute mark.
Besides, I kept it at x4 time increases because it would be fairly easy to distinguish if you kept doing something for some time, or 4 times as much. The original idea was +5 points for x2 time increase, but it was too much xP
Plus let’s not forget, we’re grounding these challenges on reciprocal trust by default so letting one decide how long their LD was would be no different from letting someone tell a semi-lucid of theirs from a real lucid. The decision is up to them, and their good heart.