When can they find the heartbeat?

I have to go for an early scan on Monday as I'm high risk and my doctor told me they probably wouldn't find a heartbeat this early, but not to worry about that because he'll be able to see if there's a viable sac. But I'll be 6 weeks by the time of the scan, and I've seen quite a few women on here post scans and mention the heartbeat before or around that time. 
So is there a definitive 'there should definitely be a heartbeat' point or it it just a case of when there is there is, it'll be obvious when it's ready? Like they might find a heartbeat at 6 weeks but I shouldn't panic if they don't because sometimes it's not obvious until say 7 or 8 weeks?