Madi-
As far as I'm aware, no.
I'm assuming that you're not planning to get a letter and store it away in your cluttered desk drawer for ten years, so you should be fine.
;p
I probably wouldn't wait more than five years before using your letter though, because I'm assuming there's a possibility that a doctor may ask why you waited so long and ask for you to reevaluated just to be entirely sure. I'm not saying this would happen, but it seems like letting your life go on and experiencing new things would make it more probable that someone questions your letter's relevance in the present if it has not been put on record before that time.
I'm actually sort of guessing here though, because not only do things vary from country to country, province to province, state to state, etc (depending on where you live), but its also highly dependent on what the doctor you see has as his/her personal requirements for HRT treatment.