How good is your line after a while, specially nylon line, depends on many factors, most have to do with the storage conditions either on line spools or line already spooled in a reel. Nylon line stored in a cool, dry, dark place lasts many years, I know it because normally I purchase bulk spools ( 1000+ yards ). Extreme dry conditions, exposure to UV light, exposure to heat weakens a lot nylon line, such thing is true either for spools as it is for lines already spooled in a reel.
Let me put you an example, I purchased by mistake 6 or 7 years ago a 6,000 yd bulk spool of 10 lbs Trilene Big Game, even though I change line with relative frequency 6,000 yards is a huge bunch of line, since I 've got so much I not only spool my reels but my compadre 's reels and several of my friends ' reels. One day I spooled a couple of Pedro 's spinning reels, two months later he complained that the line broke easily, about the same time I spooled some of my reels with that same line, I didn 't complained about the line, on the contrary, so why the difference in performance ? to make the story short, as soon as I get home I clean my gear and store it in a cool, dry, dark place, the same place where I store bulk spools, my friend Pedro doesn 't do such thing, so in a matter of 60 days the line in his reels degraded while mine didn 't.
I must say, besides storgae conditions there are other things like the ones already mentioned, line is cheap, so if you are not shure how it was stored or how much it has suffered then change the line.