That would really depend on the model and how you intend to use the photographs. If your model is with a talent agency then please consult with the agency and obtain a rate. You would first need to determine the type of photos you are taking, and in which context the photos will likely be used.
If this is your first time (i started this way), some agencies offer models who may photos for free, however you must surrender the photos to the model for their portfolio - and most likely the rights. The following are common factors that determine the rate
- type of photo (nude, studio, outdoor etc..)
- publication
- rights management.