I have only been into mp3 collecting and media center for about a year now, after about 8 months i had around 1,500 mp3 albums which equates to 18,000 songs.
For whatever reason the 1,000 album mark seems to be my threshold, it seems if i let my collection get much bigger than that it becomes cluttered with way to much stuff that i would never listen to past the first listen, therefore about every two months or so i go through my collection weed out about 500 albums or so of complete trash and just delete them, my collection always stays fresh with just my interests and always ends up around a 1,000 albums, weird but works for me.
The other thing about weeding out all one's awful albums is that it tends to concentrate the focus back to the albums that i really like and should be listening to more instead of spending all my time downloading and tagging crap that i would never listen to more than once anyway.
Granted it depends partially on how much one is spending on the music but if the bulk of it is coming from cheap russian mp3 sites or free music blogs and forums then the cost doesn't enter the picture, it's more about taking the time to manage all those files and then not being able to have the time to really listen to the music that one likes.
TC