It's absolutely appropriate to ask her. It's something you're thinking about, so it's relevant to your therapy.

I asked T what his future plans were (I started seeing him after my previous T moved to another state), because I really didn't want to develop a close relationship with him if he was just going to move or retire. He assured me that he had no plans to leave his practice for any reason. His kids aren't in college yet, so I figure he means that at least until he finishes getting them through!