I've always used a doppler from around 8 weeks (when I can first find it - yes, I know that sounds early, but it IS the heartbeat - this is DC4 - I know what I'm doing) until I first feel movement. Then movement is my reassurance because movement is, as has been said, a far more useful tool as it is the first sign of an issue.
Interesting story with DC3. I was using the doppler at 19 weeks as movement was still very infrequent at that point. The heartbeat was not its usual thumping away, instead there were loads of skipped beats (think 2-3 beats in every 5 missing at random, it was INSANE) and it sounded like there was no way a baby would make it sounding like that. I had already screened positive of 1 in 34 for Down Syndrome and was waiting it out as there was no Harmony back then, and had not yet had an anatomy scan so I assumed it was a heart condition.
Rang the Fetal Medicine Centre (one of the top fetal medicine research places in the world if you didn't already know) and somehow managed to get an appointment with the professor himself the next day and had my anatomy scan there instead of on the NHS.
Weirdly by the morning of the scan the heartbeat had gone back to normal! I still asked the professor about it of course, and he told me that this is what hearts sometimes do at this stage. I was all "Are you really really sure?!!" and he said yes - they're still very immature and the electrical signals often get in a bit of state. Even to the state I described which was frankly as low as 50-60 irregular-bpm at times. He said they don't usually worry about slow or irregular heart rates (within reason) but if they're crazy high they're actually more worried. He basically brushed off my worries in the way he does which makes you feel much much better and the anatomy scan was indeed perfect. He does not have T21 either.
DS is nearly three now and believe me there is nothing wrong with him! But hearing that heartbeat I was certain he should have been a goner.
So now I very much take dopplers with a pinch of salt. They are great for telling you that baby is still alive before you feel regular movement, but can be truly misleading (giving false anxiety and masking real issues) after that point. So now I just don't use it after I feel regular movement, I can't go through that scare again!!