Eight pounds of silica. Who cares if I have to replace every 2-3 months. I'll be good for I think a decade. Lol
h##p://www.amazon.ca/gp/aw/d/B0017JFS3M/ref=mp_s_a_1_1?qid=1427379466&sr=8-1&keywords=silica+cat+litter&dpPl=1&dpID=51u1WAywQKL&ref=plSrch&pi=SY200_QL40
This is what I use but,I can't find the 8lb bag anymore. whatever, It's 9$ for 4lbs. It has a coloured crystal that changes when saturated.
As a control I put a small amount outside the ammo can and it the blue crystals change after about a month. Keep in mind this is pretty much exposed. Once in the ammo can and sealed I'm pretty sure this stuff will keep the contents dry for a long time. I've only used it for 8 months so I can't comment on anything longer than that.