Fishing line - how do you replace - time or usage?

I usually change my mono right after I get done chunking a $15 spinnerbait 50 yards into a pond

Both

Braid I'll change about twice a year, mono I'll change after it starts getting some memory in it, which is sometimes one trip if I catch a frickin bull red.

Braid doesn't need replacing until I start to run low. It's not dependent on the amount of trips, etc... just on when the spool starts to get low. If I fish a lot, maybe once per year, but I have a reel that's had the same 30# power pro since '08 and no issues because I run it all the time with a 2 yard Fluoro leader and that's all the line I replace at a given time. Braid doesn't deteriorate with uv or heat or anything else. Braid should last a lifetime if you don't cut it off.

Braid, once a year is plenty.

Flouro, different animal. And I just really started using it this year, and I like the results. But I've probably already changed it at least twice.

It costs 11.00 per reel to change it, and for what a fishing trip costs, that pennies not to miss a fish.

Damn, I haven't changed my braid in about 3 years. I'll cut off about 10-15ft from the end every now and then