I was on Zoladex, on the side of the packet it says every 84 days. Well strictly speaking that's how it was tested and that's how it got its licence, so that is what should be done.
However in my opinion, I find it hard to believe that the little pellet can be predicted to dissolve that precisely. So I am inclined to believe there is some leeway.
I think the next question is if you can't get the exact date should you try for a few days earlier or a few days later? When I came off zoladex it took well over six months for testosterone to recover, but I guess there would be a tiny chance that the reintroduction of Zoladex if it had been allowed to get to zero, might cause a testosterone flair.
If you have the dose a few days early then you may have a slight overdose of the drug, but as I understand it, the drug just blocks some receptors and once they are all blocked the surplus drug is just processed by the kidneys. So I don't think a surplus of the drug would do any harm at least not for a few days.
So with the above considerations, whenever I could not get the exact date I always asked for a sooner appointment rather than a later one.