Mono stored at room temperature or lower and in the dark should last a long time. UV exposure and heat cause mono to deteriorate. Minimize or eliminate exposure and mono will last a long long time. Some brands of leaders are packaged in opaque envelopes, which helps with preservation. Last year I used some Climax 7x leaders that I bought in 1998 and they were still just fine with no loss of strength that I could notice. Last year I used a number of Orvis tippets and leaders from 2x down to 6x that had "use by" dates of 2001 on them. Caught some pretty nice fish with them. They were all fine too.

A customer came into a local shop one day while I was there. The customer complained that all of the leader brands they had tried, which included S.A., Orvis, Climax, Rio, and Cortland, were brittle after a few weeks. After some discussion we determined that that customer always kept their stuff ready to go in the trunk of the car. Their fly lines didn't hold up long either.