They did not lie. CMC sometimes has low quality discs, but most CMC discs are good or very good. Good CMC discs do not degrade quickly. 40 years @ 25ÂºC/50% relative humidity sounds correct.
If the degradation is accelerated because of a bad disc (data is exposed to air and moisture): This kind of failure is VERY fast, but is very obvious.
Otherwise, with a good disc: make sure the temperature does not change very quickly, do not use the discs very often. I think you can get 20 to 25 years @ 36ÂºC/65%RH without issue. After 20 to 25 years @ 36ÂºC/65%RH, degradation might start. I do not think a disc would fail until 30 or 35 years @ 36ÂºC/65%RH. Most discs will last longer.
Because the temperature can drop to 30ÂºC, I do not think temperature will be a big problem. Keep them in an insulated case, and this will keep them a little cooler.