It depends. my midwife found it with the doppler at 11 weeks, but the ob registrar at the hospital couldn't find it at my 15 weeks booking in appt, so had to get the portable u/s out and check it that way - amd it was fine.

It can depend on the position of your placenta as well, if its anterior (at the front) sometimes it can take longer for the hb to be audible with a doppler.