Back when I fished 100+ days a year, I'd replace mono when I'd broken off/cut off & retied enough line to where it affected casting. (Lots of rocks and timber) Modern mono is a lot less effected by light and time than most line manufacturers would have you believe. Braided line seems to live almost forever. I've still got a few miles of it on bulk spools, including some of the old Lynch Braid, which is over 20yrs old. When you hook a log or rock with that stuff, you drag it out or the hook straightens. I believe you could tow my bass boat with that stuff! If you fish a LOT and your rods lay out on the deck exposed to the elements, then once a year or so I'd change the mono. I should mention that I use a backing on my larger capacity reels. Less line to replace, saves some bucks.
Savage