I think it's something to discuss with your T and hopefully you both come to some agreement. It is a professional as well as personal relationship and they are providing a service. If we make them work "overtime" by using them off hours more than necessary, I think that they need to be compensated, as we would, in our jobs if we were working more than we got paid for really. Although yes it's not a 9-5 job but if we are using them a lot after session then compensation might be helpful to bring up.