"After such an injection, the level of hCG in the body will fall by half every 36 hours. This means that after an injection of 5,000 units of hCG, enough hormone will remain in the body to show up as a positive reading on most pregnancy tests for eight to 10 days"- ivillage.com
Hope that helps. I had also heard wiating 2 weeks just to be safe.
I was told that it is out of your system in 10-14 days, so it's best to wait the full 14 before doing a HPT.