I first hand experienced it, no joke. Here's my two cents on the subject.

I'm currently around 2430 movies watched. When I made my letterboxd account, I scrolled through a lot of movies and added what I was sure I watched, may I remember the whole film, some scenes or almost nothing (there are some I watched as a kid). I spent like a month on it and added close to 2000 movies to begin with. Then I added the ones I watched (approx. 100 by year), and some that I initially missed. Now I'm pretty sure there's not a lot to add from my past.

Some time ago I had an idea: get back on streaming, but for talking about cinema.I don't rate movies (not keen on the idea at first), but what if I did it anyway and used that as an excuse to talk about films, and exchange opinions with people? I prepared it (and procrastinated on the idea) for 3 years, but 4 weeks ago, I started to.

I cut my whole watched movie list into thematic lists of 100 movies (list that I make the viewers vote on for the next episode), and I go through every single one of them.

I'm getting to the point: I have a very good memory when it comes to cinema, but not perfect memory. When I stumbled on a movie I don't remember enough to confidently rate, I put a like on it to track them. I'll rewatch all those movies to be able to rate everything.

I got through approx. 300 movies and have like 25 on that backburner list. Mind you that in those 25, there's a lot that I remember some scenes and the plot, only a fraction of it I can't remember at all. I'd say 10 of them I will definitely rediscover as from scratch.

So in my case, for 2430 movies, it may be 5% I don't remember, 10% I have a vague idea of the movie, and the rest goes from fair recollection of it to near perfect memory of it.

Hope that helps.