If line is properly stored (for example, dark and dry), is there a general rule of thumb as to how long line should last?  How long does it take for it to weaken or deteriorate, if at all?

Thanks

You are going to get a lot of different answers but I have some that is more than 5 years old and it is fine. There is a little bloom on the O.D. but I've not had a problem with knot strength or anything else.

I haven't had problems with line that's years old, I don't know many years, though. But don't store it where it's exposed to the elements.

Think of it this way, you don't know how long the line has been on the shelves or storage areas of sporting good stores.  But I always spray new spools of line w/ Real Magic or KVD line and Lure as soon as I buy it and about once a month during the winter.