I used mine daily from 15 weeks but only for a min. When you start to feel the baby move you tend not to need it any more
I've read that you should try to limit it to once a week or so. The home dopplers tend to be stronger than the ones the doctors use so they think they're potentially damaging, if used too much...I'm not sure if they know anything definite, I just limit myself to be on the safe side.
I used one with my first son pretty much daily from 14 weeks up until a week before he was born.. I wondered if it could be harmful as well so I researched it online and couldn't find anything saying it was harmful.. and my son is almost 2 now and perfectly healthy! :)