Tough question
I'll try to get you some facts, but it's hard to answer, mainly because there is no accepted definition of "Breakdown". I didn't mean to imply that in 40 years fluorocarbon will be gone, just that it'll start to suffer seriously... try centuries for "ashes to ashes, dust to dust" time... Mono begins to lose its integrity the second it hits water, or is exposed to UV or heat. It becomes weaker and starts to "breakdown". The industry advocates say this makes it less of a danger to wildlife, boating, etc. Find a wad of mono some weekend baitchucker left on the bank from last season when he suddenly felt the need to peel off 50 yards. It's chalky, brittle, easy to break. Fluorocarbon on the other hand is much less affected by water, and by UV. You can use a 5 year old spool of fluorocarbon you've left by the window with good confidence. As to throwing mono away, most bait shops will accept old mono for manufacturer sponsored "recycling" programs. Save it up and bring it in, don't throw it away. Don't mean to attack you personally. You may not be leaving alot behind, but you aren't alone. A couple hundred of "you" are leaving a negligible amount behind, but a couple of thousand are leaving alot more behind... and that's just on your favorite river. I once snagged what looked like 80 or 100 pound mono on the Skykomish, started to wrap it around my hand to pull it free, when I started to feel a pulse. Got it off just before a huge salmon jumped and took off upstream, taking the line with it. I'd hate to think what kind of damage I would have suffered if I had been slower. I also hate to think what will happen fluoro gets cheaper and A-holes who would fish that way can afford it...