I know durak didn't say to use Silica BB's or anything, he was just telling you about the different materials but NO! NEVER USE SILICA BB'S. They are for the most part banned at most fields here in the UK (and I assume the rest of Europe).
Apparently they also put extra stress on PTW's and you can seriously damage your PTW if it starts to chop since silicas don't actually shatter without a lot of force whereas standard styrene ones will shatter into "confetti" if the PTW starts to chop the BB's.
As for BIO BB's, in optimum conditions I'd say more like 3 months to degrade. In realistic conditions however probably a year, possibly more and in regular storage conditions you're probably good for a year and a half before you should think about throwing them away. Also it might be a good idea to toss in one of those silica packs or some rice grains to soak up ambient moisture so the BB's don't degrade as much.
In short, stick to the tried and true and go with styrene if playing unless the field stipulates Bios. And only use silica's for target shooting (they shatter glass and can shatter other players goggles resulting in safety issues which is why UK fields ban them from use in game).
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE