When I got mine around 15 weeks I'd use it every other day for a couple of weeks. But always at the same time, before bed, kind of like a 'goodnight Bump' thing from me. And then I'd find the heartbeat and only listen for 10-20 seonds, no more. If I could not find the heartbeat within a minute or two, I did not persist, but let it be and didn't worry.
I'd never listen if I had something like tummy ache as I think that just leads to more worrying tbh. Using it same time before bed meant I didn't worry if I could not find a heartbeat one time, and did not fret. Whereas using it if I had a pain or something and then could not find a heartbeat would maybe cause me to think more irrational thoughts and worry.
Once I got to about 18 weeks and beyond I went down to once a week. Now, since about 22 weeks and baby is kicking a lot, I hardly ever bother.