It would be great if you could put a dynamic Review button in the client appointment booking page to denote whether this appointment should be the Last Review Date. Also if there is a field that says Target Date of Next Review you could enter the date specifically or choose 3, 6 or 12 months interval and it would default to a date in the future so you could run a date based report to remind you who to reach out to schedule. You could also run a Last Review Date Report but these would each be a way to help project what the future volume of appointments looks like.