Why does the glue in a puncture repair kit dry up so quickly?
In my experience:


You only need to apply a drop of glue a bit larger than a pea (about 7mm diameter) per patch;
Glue inside eventually dries. If you only have a puncture once a year, most probably the glue remaining from the last puncture would be dry "no matter what". Also, there is an expiration date of around two years, but I think it's two years if you didn't open it yet;
(IMPORTANT) When you attach the cover of the tube, BE SURE TO REMOVE ALL THE AIR INSIDE by pressing its bottom untill half a drop shows up in the opening, otherwise the glue will evaporate inside the very tube (happened to me a lot of times until I figured it out...).
These glues have a volatile nature. I wouldn't buy it in large amounts, but rather to buy new spare glue tubes (without the whole patch kit). The kind of patch I use also have this problem: the patches (a small roll I have to cut with a scissor) last much longer than the glue.


Well, hope it helps!