I fish only fresh water, and usually only twice a season.

I usually respool about once a month but thats because I do a lot of light line fishing for big fish

I have lots or rods and reels and use them at differing levels. Those that get hundreds of hours of action each season get replaced a lot more often than those used every three years or so. I usually re-spool all but the 60-120 pound offshore lines about every year or two depending on what the line is doing (I have 5 year old line on some of them as long as the line is good). I replace light casting lines as soon as I notice any loss of line flexibility (the line starts to retain spool loops rather than straightening out nice and smooth). I don't worry about this issue for trolling lines since they don't have to cast. I test ALL knots for strength and I replace the casting or trolling line as soon as I notice any brittle line that causes a knot to break. I NEVER fish with a weak knot no matter what. When head count matters, I test the last 10 feet of my line by running a nylon stocking over it to detect cracks and nicks every time I catch a fish; otherwise I do this before I put away the rod at home). I cut off any deffective line immediately as soon as I detect a ding. I guess I am either cheap or very conservative but I only replace line as necessary. I know that line manufacturers and their sports writers advocate more purchases but I have found it is just fine to do it only as necessary. I have caught thousands of fish from bream to marlin for the last 55 years and can not recall losing one to a broken line (except one 450 pound blue marlin that flipped backwards and cut twin 1000 pound leaders with his bill); nor do I ever allow line problems to hamper my fishing. I highly recommend watching your knots and line religiously. If you do, you will know when it is time to replace it.