The characteristics of Androgel
indicate that the steady state blood level is achieved after 3 days of use.
In the steady state condition (i.e. what blood levels are like after prolonged use), there is a small peak of testosterone
during the first four hours of application, then levels are generally relatively flat thereafter for the rest of the day. There is a small dip at 12 hours after application for the 10 gram per day dose, which is not present for the 5 gram dose, but I think it is relatively minor.
Probably it would be good to get the blood test
before Androgel is applied that day or at least 4 hours after application.
The characteristics of Androgel also means it is possible to get a blood test
after at least 3 days of use of Androgel to see where one is in regard to only testosterone levels and absorption.
However, I would wait several days longer to allow all the other neurotransmitters and hormones to be adjusted to the change in testosterone, if measuring them is of interest.