I would definitely recommend an expiration date on person training sessions/packages. You can implement two different methods:
1. Have different expiration dates which depends upon the amount of sessions someone purchases (i.e., 4 sessions expire in 1 month, 12 sessions expire in 4 months).
2. Set one expiration date for any amount of sessions purchased (i.e., your sessions will expire in 6 months).
Either way would be good, and like LaRue said this will help build consistency with your clients. Also, spell out any type of cancellation policy, expiration policy, and payment policy within your waiver/agreement so everyone is on the same page. One thing that I do is include the expiration date on the wavier/agreement that the client reviews and signs, and I give them a copy as well.
Hope this helps!