well personally I don't think there is a way to set a limit on how much you can imagine I think it just depends on the person, like I've met some people who can barely picture a rock in their head but some people I know can see universes and other planets and can describe a very convincing alien, so honestly I don't know what my limit is.
Imagine how boring it must be to only be able to picture a rock. I’m able to have a lot of fun with my imagination, creating whole world and universes in my head from scratch. Endless hours of fun if bored
Have you ever head of something called Aphantasia? People who have it, to varying degrees, have no visual memory whatsoever. They're incapable of imagining anything with a visual component, for some it extends to audible memory as well. They wouldn't be able to remember what your face looked like to draw it, nor do they necessarily dream in the same way others do. A friend of mine can't recall music, if you were to mention a song to him, where you'd normally be able to hear it in your head, he can't do it. When he reads, he doesn't imagine what's being described. However, he does suffer from night terrors, which is interesting, since that does involve imagination. So, it's not that the imagination is the problem, it's that a very specific mental state is necessary for it to be possible, the problem lies in voluntary visualization. People who have this can go for years not even knowing they do.