You are always invited for a follow-up, but you don't have to do anything about it. A follow-up is not compulsory.
If your treatment worked and you have no more questions, simply ignore the invitation. You don't need to let us know either, and nothing expires.
If the invitations bother you
Let us know and we will stop them. That is a perfectly reasonable request, especially when your treatment finished a long time ago.
When a follow-up is useful after all
Your complaint has improved but has not gone away completely.
You are on a longer treatment and your dermatologist mentioned a check-up.
You had side effects that you have learnt to live with.
In those cases, a follow-up can make your treatment shorter or easier, and that often delivers more than it costs.
Coming back later
If your complaint returns in a few months, start a new consultation. Your file is kept, so your dermatologist can see what you were treated for and how you responded at the time. That makes the second assessment quicker and better than starting from scratch.