What doublebicep said, plus, it will not affect your natural test production, because at this point I assume you've been on TRT for a while, and in that case your natural test production is shutdown anyways. HCG will help re-start the HPTA.
I honestly don't know the answer to how long can you use it. However, I can tell you that in the case of an anabolic steroid cycle, it is recommended to follow that same HCG regime throught the entire cycle. I've done it, with great results. However in the case of a TRT, with small doses of test, I am not sure if the same dose of HCG every week could cause some desensitation of the testes. Another thing is, some people see increased estrogen levels from HCG.
In this case of a TRT, HMG would be a better option. There is still too little experience with it, but from what I've been reading and researching, seems like 37,5IU's of HMG twice a week is the way to go. HMG does not raise estrogen like HCG does, and stimulate not only LH but also FSH.