It is your decision when to stop - you are her guardian and she is a minor, and you pay the bill. But I'm sure you don't want to stop it if she really does need it.
What is your daughters opinion? What does she think she gets out of it?

The therapist cannot tell you about sessions but if there is a good reason for your daughter to stay, the therapist can discuss with your daughter what she can tell you - it doesn't have to be any details but enough to put your mind at ease.

Like sunrise said, can you go to a session together to discuss this?
Or how about start spacing out the appointments and see how she goes with less frequent sessions.