If I understand correctly, the optimal way to use the optimize feature in FSRS is to do it after X amount of reviews (say 5-10k).
I have 6 decks with very different properties/characteristics and # of cards. Some of them will take over a year to get to 5-10k reviews while others will get that in a couple months.
Anki should have some sort of record that shows how many reviews I have done in the deck since the last time I optimized so I can know if I’ve reached a suitable time to reop.
It’s pretty easy to just do it monthly. When you get to the point where FSRS can’t optimize anymore, it won’t hurt to check.
Honestly I don’t think the user should even ever have to hit it. It should have a setting to optimize every X reviews and automatically optimize.
There are a few issues with automatic optimization – but those have been discussed extensively, with great passion and rending of garments, in other threads, so I’ll let you look those up if you want to explore that issue more. 
