I'm sure pollen cells can live upto 12 months and still be viable.. I've heard of people placing them in the fridge/freezer and still having them work after taking them out.. I think them being in the fridge makes sure no bacteria can kill them..
Well nobody knows it for sure, please note that when the stigma is "open" to receive the pollen then the anthers on this flower slope down and the pollen on them loses its viability within a few days. If collected, it can be kept for several months in your fridge. I used this method when I wanted to obtain some crosses and the plants were flowering at different dates. I used to cut off the whole anthers and keep them in a small piece of paper, wrapped with tinfoil. It's also a good thing to cut off the anthers of the flower on which we want to use our pollen, to avoid self-pollinating.