How many hours should I leave 5% minoxidil on my head before showering?

Ideally it is left on for 8 hours but the 'minimum' time I advise is 4-6 hours. After 4 hours, 75 % of the minoxidil is absorbed.This advice comes from a research study back in 1990. In that study, 22 healthy male volunteers completed a four-way, multiple-dose, randomized crossover study to determine the relationship between contact time of applied drug on the scalp and minoxidil absorption from a 2% topical solution. The researchers examined how much minoxidil is absorbed after 1 hr, 4 hr compared to nearly 12 hours. Relative to the amount absorbed after a contact time of 11.5 h, absorption was approximately 50% complete by 1 h and greater than 75% complete by 4 h. 

You can use Minoxidil twice a day, once in the morning and once at night.  You can leave it on all day to let it absorb in the effected areas.

I recommend using it after a shower and leaving it on all night so that the drug absorbs in the skin to get the best effect. Shower in the morning