You would normally opt to use HCG after you've done a long cycle, usually 8 weeks or more.
In any case, you should run it about 3 weeks, totaling about 4 shots. One every 5-6 days. Start off with one shot of 3000 IU somewhere in the last week of your stack, then another 3000 5 days later, then drop to 1500 5 days later and a last shot of 1500 6 days after that.
Sometime after the second or third shot, therapy with Nolvadex or clomid should be commenced and continued for 4-5 weeks.
HCG IS and always will be an important part of post-cycle recovery, but it should never be run too long or at too high a dose and should always be accompanied by the use of either Clomid or Nolvadex. The use of Clomid or Nolvadex should also be continued at least 2 weeks after HCG is discontinued to avoid the HCG causing problems.
My preference is for Nolva + proviron instead of clomid.
Week 1 PCT: 40mg/day Nolva, 50 mg/day Proviron
Week 2 PCT: 20mg/day Novla, 25 mg/day Proviron
Week 3 PCT: 20mg/day Novla, 25 mg/day Proviron
or you could run clomid as you mentioned:100 mg/day week 1 and drop to 50 mg/day week 2,3