Yes. There should be an expiration date on personal training sessions.
Here is my rationale.
If they are seeking your services, they obviously want some health, fitness, performance outcome. The science is clear on what is necessary in order to effect such. That needs to be explained during the subjective assessment. Too, readiness to change an unwanted behavior has to be assessed.
If an individual is taking forever to finish a a package of 10 sessions, it is indicative to me that they were not in the ACTION stage of change.
On another note, Karin’s suggestion gave me a good idea. You might think of putting packages together that a suitable to an individuals stage of readiness. In this way, you don’t have to have clients with sessions that have exceeded their sell by date.