There are LOTS of women who don't hear a heartbeat until later on. 6 weeks is standard, but some may even be 8 weeks before the heart starts beating.
This is NOT wise. There is a very little time frame for the heart to start beating. It happens late in 5+ weeks into early 6+ weeks. If there is no heartbeat at 8 weeks, then the pregnancy is not viable, period.
If there is no cardiac activity by, let's say, 6.5 weeks, it is highly unlikely it'll start. An hour can be a huge difference at this stage. If you go in at 9 am at 6w1d, there may be no heartbeat. If you go back at noon, there might be. Saying that it might not start until 8 weeks, however, is just plain wrong information.