If you find a spool of monofilament back behind some stuff and you know it has been there for at least 3 years, is it still good?  What is the shelf life of monofiiament?

As long as it’s kept from UV radiation ( direct sunlight ), ozone, extreme dryness nylon lasts a very long time, I have spools of 10 years old nylon and it’s perfect.

 

UV or sunlight and temperature extremes from freezing to over 105 degrees degrades mono.

Stored in a cool dark area 3 years wouldn't degreed it.

Tom

I used 8# XL that had been on a spinning reel for several years to land my biggest ever catfish (20 lbs.).  Raul and Tom both know of what they speak. 

If this will help, I have a jumbo spool of BPS 20lb Excel on my fishing peg board next to my work bench downstairs, which I only use on one  reel and change it every year and it is now 18 years old.