“I don’t really SRS anymore” is something one occasionally hears from advanced learners, usually with a twinge of envy. Compared to reading, watching movies, conversing with natives, SRS feels like (and is) a pretty dry and mechanical thing. Some days it would be a nice relief to drop it entirely. Of course no native speaker uses SRS for their own language.
Then again some will insist that one should never stop SRS, even when the card intervals stretch into the years and no new cards have been added for a long time.
It’s actually very easy to tell approximately when you can permanently delete or suspend any given SRS card. SRS is designed to remind you of a fact just before you forget it; so if you see the fact (be it a grammar point, word, or kanji) in the course of your normal day-to-day use of the language with a greater frequency than the card interval, you don’t need that card anymore. In fact, keeping it around is a waste of time. You can see that this is always going to be approximate, going by intuition not exact statistics.
Thinking of it mathematically like this it’s obvious that an advanced learner will both have his average card interval much longer than a beginner, and will see the facts on those cards far more often, due to faster reading, better listening comprehension, and involvement with more advanced materials. Then the time inevitably comes when new material comes in so infrequently, is so easily remembered due to the massive context already absorbed, and old material is so ingrained that the whole SRS process can be dispensed with. You could graph the whole process if you wanted.
This is partly why I like to keep separate decks that are at least somewhat homogeneous. It is a time saving to suspend an entire deck of easy grammar, for example, rather than continuing to review that deck and suspending cards bit by bit. One also needs to learn to take a somewhat detached view of the individual cards. A dragon might know the details of every treasure in his pile, but it’s a bit much for a human; and anyway there’s so much treasure already there and flowing in all the time that a gem or two or a dozen will never be missed.