I have had early scans with all my babies. Earliest was 5+6 and saw a tiny little blob with a hb. This time we had one at 6+5, tiny baby seen with hb.
The earlier you have them done, the more chance there is of you not seeing much, so definitely after 6 weeks, the closer to 7+ weeks the more chance there is of seeing a hb, but still sometimes you have to wait a little longer