If my memory is correct, its about 3-6 hours once they are activated before they downregulate. The presense of tesosterone will prolong their life somewhat.
I may be wrong on the exact time. If I remember correctly, it also varies signficantly from one tissue to the other---testicular tissue, the seminiferous tubules for example will have different receptor lifespan from other tisses just as an example. I'm going from memory here......