Changing your mono once a year is a bare minimum. Wear can require changing far more often. I have a LOT of rods/reels, some of which I rarely use. But If I pull out a combo that has last years line on it, more often than not, the line has "memory". It has been on a small spool for "who knows how long" and if you cast it, it will come off in coils. Knot strength is also dimished because it's dry. There are exceptions to this, of course, but as an earlier posting said, "don't let the line be your weakest link".
Braid is a different story. I have one reel with Fireline on it that will be entering it's fourth season. I want to see just how long this stuff holds up. While it has lost it's coloration, it's still plenty strong. I won't change THAT reel until the line actually deteriorates.
Needless to say, a carelessly discarded tangle of line can be a deadly hazard to wildlife for many years, make sure it's not yours in the rocks!