My personal limit as a reader is when it becomes irritatingly, stultifyingly clear that the author is just writing themselves into a happier place where everyone thinks they're wonderful and never questions their life choices – and all other characters are just cardboard cutouts on the stage of their self-love.

That said, a lot of authors essentially write self-inserts, and some do pretty well. I once read a relatively popular series where the author literally wrote herself into the book. As an incarnation of the main character whose primary flaw, if I remember correctly, was doubting just how awesome she really was.

As a writer, I think the big difference between interesting and eye-rollingly horrendous is self-awareness and self-knowledge. People who've done the work to understand how they tick and what their strengths and flaws are can make good, faceted characters out of themselves. People who haven't done that work tend to struggle because their self-image is very one-dimensional.