Doppler use

So, I used my doppler super early to find baby (7+5 and 8+0). It took me FOREVER. I used it 30 mins. the first day, the 45 to 50 the second day. I never used it that long with my previous pregnancy. Had anyone else used one this long before? Could I have hurt baby? It was only on baby 3 to 20 seconds where I actually heard baby...