Definitely pre cycle so you know your baseline levels, and then a few days after PCT for sure to check for suppression, e2, hemo levels all that good junk. Mid cycle like kbmxxx stated isn't a bad idea either, although depending what your running you should be able to tell if you got bunk gear or not pretty early on. If your cruising (which is my only real experience so far) I get mine checked every 6 months after everything has stabilized and you have found your correct range and dosage.