It depends on how you frame it and advertise it, if you sell it as a gift card or gift certificate (or it could be considered as such, based on how you sold it) you may have to honor it for up to 5 years based on the CARD act passed by the government.
If instead you frame it as a contract for services and allow them to choose any dates within a *clearly* specified time frame you would generally be in the clear.
In some states though you are obligated to refund the sessions that expire. You can’t collect funds for undelivered services. Groupon just went through a large class action suit over this.