Here is how I see it----when it is something like aero, that doesn't change with physical conditions. if it is 80 degree or 108, the aero is the same. Same as fatigue---the aero advantage is the same no matter how tired the rider might be.

As for practical gains ... you would need to do a ride every morning at the same time and about the same temps, on the same route,probably at least six days ... one day each in each setup, three times, to get valid Practical data--as in, how much time will each helmet take off my ride time over that route..

However, if all you wanted was the difference in practical drag ... as in, in my basic aero tuck position, how much would one helmet benefit over another---that could be measured almost instantly. The aero difference is never going to change. it is a function of the shape of the helmet as it sits on your body in your tuck position. Five minutes with each would be plenty ... much less might be better, because a one-minute sprint would give all the needed data for each helmet, and you could rest half an hour between efforts to be sure each was a fresh sprint.