How often do you replace your fishing line?

I have a few reels that the line (Berkley Nanofil) is several years old.  How often do you guys replace your line?

My braid rods all run a 6ft flouro leader so i only have to change that line every couple years because there is not much cut and retie with the actual braid. My flourocarbon gets changed yearly or more. Depends when it starts to cast bad.

All of my rods with braid get re-spooled probably twice a year. Mono and floro probably once every other month. Trolling rods with mono get replaced when my baits aren't running to the desired depth (usually twice a year). This is due to break offs or frays in the line or nicks from weeds, rocks, or other structure. 

I always have a backing on all of my reels so I'm not filling up a spool and only using a quarter of it then pulling it back off.

I might add that I fish rivers a lot and also in waters with a lot of rocks and trees/wood. I fish open water all year so my gear gets more of a work out in a year than most guys' gear will see in a lifetime. Nothing gets me more upset while on the water than losing a fish due to line failure.

A bit of advice when spooling braid. I seen this on a fishing show this spring. If you have trouble braid twisting, with the line spool set vertical try spooling the line from the bottom of the line spool and not over the top, the guy on the show says it works but he couldn't explain why.

I wouldn't replace Nanofil unless it started to frizz,same with Fireline.Mono and such,I replace when needed.Once a year,unless it has seen abrasion or excessive sun exposure.UV kills monofilament/fluorocarbon lines faster than you think.Like those guys that leave their rigs in boat,exposed to the sun for days. 

I replace line every spring and as needed throughout the year but I always leave 1/2 spool of the "old" stuff on the reel as backing.  Don't have to buy so much line that way...saves a few bucks when changing line on 12-15 reels every year.

Cotton wood is easy to get off.Pull mono tight and snap it like rubber band. Works better dry than wet but still works both ways.

As others have said, it depends on the reel and application, and frequency of use.  Am I fishing open water trolling, zebra muscles,  rocks etc.  My spinning reels with mono, more often, some of my reels with braid, less often.  One thing I do though is on my line counters I do not replace all the line.  I'll strip off 300-350 yards and refill that much line.

Thats what I do,I just pick it off,if I did not do that I would be replacing line every three hours.That said when all I used was mono I did have to replace mono almost every week of solid fishing.

Some rods that i run mono/floro/copoly I have to refill 3-5 times a year.  I fish rocks a lot.  If you lose 6-8 jigs every time out you're refilling in couple weeks.  I run backing on every mono/floro/copoly.  Old line casts like crap anyway...and once a line gets a lot of twist,  no matter how much you get twist out it seems to come back faster.  It's gone,  respool. 

When the cottonwood flies in June,  I respool almost daily for a week.  you can't cast a rod with that crap in a spool,  a couple pieces and you're done.

Ive fished w people many times and they have a half spool of line. they can't cast for crap and they wonder why.  I don't care if it's brand new a week ago,  if it's too low on that lip, you change it unless you are just vert jigging and don't need a full spool.

PowerPro/braid/nano - when the spool gets too low and I can't get max distance on a cast.  usually every other year.

Mono and copoly is cheap,  if it's 1/8 below the lip it's too low.  If it seems to be holding a ton of memory, lot's of twist, change it.  It's just my feeling on line,  cheapest part of your equipment and one of the most essential.