It's something you can worry about, but you don't necessarily need to. When I have seen therapists, one of the things we discussed at the first meeting was our projection for how long the engagement would be. I asked, and was told that in most cases, both parties reach a point where they feel that further engagement is not necessary. And that's exactly what happened. We met twice a month for about 4 months, and after that, we both agreed that things were improved, and I had built strategies to help deal with my issue, and I stopped.

It's a concern, but one that can be addressed head on. Many good therapists are over-booked anyways, so their incentive to keep you on is low, as there are other people who want additional appointments and can't get them.