[I never did a systematic clean out of my things and probably never will.]
A few years back I could never imagine throwing out (i.e. donating) any of my books, but when my bookshelf didn't fit into my new room any more I managed to throw away the books I didn't like. That was a huge step for me (any normal person would ask why keep books you didn't enjoy, but hey that's me).
A couple of years later I'm now at the stage where I got rid off some books which I read, but couldn't remember any more and knew I wouldn't read again (so they didn't make a lasting impact on me). I feel comfortable with this: Keep the books I enjoyed or which have sentimental value to me and throw out the rest. But I can even imagine that in a couple of years I'll just keep my absolute favourite ones.
The important thing is that everyone's different and everyone has to do whatever is comfortable for him and at his own pace.

That's exactly right. It's really whatever someone is comfortable with. There is no right or wrong; a minimalist is not morally superior to a hoarder.

I never would've imagined I'd be happy getting rid of any of my books, but then it turned out that I could let some go and feel good about it. I suppose mostly because I now have room for new books, new discoveries, and hopefully some new absolute favorites.