I'd like to buy some line in bulk and just keep it down in the basement. Does anyone how long the line will last when it is stored properly? 

When stored properly - I'd say indefinitely.

Or at least until I can't cast any more.

A-Jay

Sweet. That's what I kind of thought. 

2 things tend to shorten monofilament line life; direct sun light and freezing or high temperatures that exceed 105 degrees. Kept in a cool dark area it will last a very long time.

Tom

My largest fish ever was caught on 8# Trilene XL that had been on the reel several years.  I've got 3000 yard spools of 8# and 12# Silver Thread AN 40 that I figure will still have some line on the spools when I die.  I've never worried about Nylon lines deteriorating to the point I couldn't use them.  I change out line when the spool gets too far down....or in the case of spinning reels (normally) when there is more line twist than I feel like dealing with.  Don't let anyone tell you that you can't get a severe case of line twist with a baitcast reel.   They just aren't fishing the right lures tied direct.  A lesson I learned the hard way.  At least the braid had been used for a few years before I did it. 

I've had 1500 yard spools of Stren in the basement for over ten years and it's as good as new.

I have a couple 1000 yard spools of Berkely Big Game that are several years old and comparing it to a new spool, there isn't a difference. Still fish with it to this day.