If you read the package insert sheet, levels can be checked anytime after 2 weeks, IMO, the sooner, the better. Steady state levels are reached in a couple of days, so there's no reason to wait.
The only 'clinical' studies appear to be ones done by the Axiron people which doesn't give me a whole lot of confidence in the product.
Tried it for 2 months, eventually upping the dosage to three applications (180 mg) per day, but with little success. Total test was not too bad at low mid-range, but as expected, DHT - off the scale.
Considering the cost and marginal effectiveness, overall I was not too impressed, however, as with any transdermal product YMMV.