I used Pregnyl and bought a whole load of cheap pg tests off the internet, which I used each day after the shot, so when they came up neg, I knew the HCG from the Pregnyl shot was out of my system and any positive after that would be a true positive. If I remember correctly, the HCG was gone after 7 or 8 days with me, but I know it can remain for longer in some people. If you're not going to test manically like I did - then it's best to wait until 14 days after the shot to be truly sure that it's gone.
Good luck!
Jayne x