What brand are you using?
From what I've heard from word of mouth they still take a long time to degrade. I have some Tamashi's kicking around that have been in an opened bag at different heats and in direct sunlight and they don't seem to have changed... and it's been about 3 months.
To prolong their life keep them in a sealed black back out of sunlight for most brands UV rays, water, co2 and other gases and elements are what degrades the BB's.