Depends on how big the site is, but I do go well over 500 lines on some sites, although I do break each declaration into a separate line, which I'm reconsidering at the moment.
I prefer one style sheet over multiple ones, but I know others have a different approach.
In the end, I would say it's not so much about how many lines of code you have, but about the size of the CSS file in KB. I've heard some opine that you should aim for a maximum of 12KB for the CSS file, but I don't know how many people subscribe to that view, and I know many style sheets go over that.
EDIT: regarding long style sheets being hard to manage, I find that using a tool like Firebug makes it easy to find the relevant line for each declaration, so I never have trouble finding the right place in the style sheet (as long as there are line numbers in the editor).