I would at least keep it on for 2 hours if not until you started feeling something if I where you, but that is just me. Honestly though I doubt you will end up getting it onto someone else or yourself if you are careful and aware about it. If you are testing it you want to thoroughly test how long it takes to fully absorb as well as how much so someone doesn't go higher and leave it on longer getting an extreme dose as he though all of it would absorb within a short amount of time even if you mention you washed your hands as someone may base their idea of a fact not on a study, but the fact you decided to wash your hand within 30 minutes and assume that was based on something and assume they do not need to wash their hands after dosing.
You want to make sure you test it right and get full results so that someone does not try to get a stronger effect by using way more than you do and end up hurting themselves because they did not reproduce your procedure to a T and leaving it on to get maximum results rather than washing it off. Basically if it is able to absorbs, but is only able to at a slow rate (like 5 MCG per minute than) it may take longer to get larger doses in essentially. With my random number it would take 30 minutes to get 150 mcg absorbed, which is active although as a light or threshold dose.
Just be careful when you do it and dont shoot to succeed the first time by using a high dose, but do it carefully over time with many trials where you slowly and slowly build the dose. (50 mcg first time, 100 mcg second times, 200 mcg third time, 300 or 400 mcg) not exponentially, but by a set rate once you get to higher doses like 25, 50, or 100 mcg increase per trial. Good luck on testing this though and be sure to stay safe.