I found this information from one of our sponsers on this board. I figured it would be good to post since all of the threads popping up about it lately:

Based on studies with normal men using steroids, 100iu HCG administered everyday was enough to preserve full testicular function and ITT levels, without causing desensitization typically associated with higher doses of hCG. (2) It is important that low-dose hCG is started before testicular sensitivity is reduced, which appears to rapidly manifest within the first 2-3 weeks of steroid use. Also, it's important to discontinue the hCG before you start Post-Cycle-Therapy so your leydig cells are given a chance to re-sensitize to your body's own LH production. (To help further enhance testicular sensitivity, the dietary supplement Toco-8 may be used)
Taken from this article. A good read:

Health and fitness articles: HCG Diet Information