How Often Can you Use a fetal doppler?

I went and bought a fetal heart doppler / heart monitor from ebay. I have tested it and it works but how often can i use it in a week (not that i am going to use it weekly just as an example of how often i can use it) , i dont want to over use it because i dont want to hurt the baby. Has anyone else had experience with these heart monitors and how often you can use them ?