I've got about 43,000 songs in my collection (on a music server). It has been compiled from CDs I've bought along with LPs and open reels in my collection from years back that I've converted.
For me, the point of that size collection is not that I am constantly listening to every song repeatedly, but rather I have an enormous selection of choices available for whatever mood I'm in.
Not too long ago I went through a spell where I was listening to a lot of stuff from the 40s and 50s - Louis Jordon, various boogie woogie artists. Other times I may be in a mood for classic jazz - Chet Baker, Coltrane, Art Blakey. Or perhaps I'm exploring newer releases by Diane Birch or Gogol Bordello. I also love classical which offers an endless array of recordings for exploration, especially if one can appreciate having more than one version of a piece or is interested in going beyond the common standards.
I know some people primarily stick with mostly one genre and have their solid preferences when it comes to artists. However, when one is a musical omnivore, the collection tends to continue growing and growing. I wouldn't have it any other way.