I have an amaryllis plant that is getting ready to bloom, and I know that I won't have enough time to cross pollinate it with a snowdrop so I want to know if it's possible in general to preserve pollen for extended periods of time and if so, for how long?
Like you said, flowers for cross-pollination don't always coincide. Here's how to harvest and save pollen.
After the flower is open, the anthers have burst/opened, and you can see mature grains of pollen, take a fine brush, and gently remove some of the pollen and put it onto a white sheet of paper. You can use pollen from more than one flower, as long as it's from the same plant.
Place the paper in an area of warm temperatures and low humidity. It should be very dry in less than 6 hours. If there is no dry place, put it in a small sealable bag with a desiccant such as silica gel, and put that in a sealed container (such as a canning jar, or tupperware container) and set in a warm place for a few hours.
After that, fold the paper containing the pollen, and place it into a small, dry tightly sealed container with a desiccant (contained, not loose). Place this in an area between 32 and 40 degrees F. until use.
When the flower you want to pollinate opens, take the desiccated pollen, and apply it to the receiving pistil(s) with a fine brush. Then raise the humidity, without directly wetting the pollen.
After a few months, the pollen may lose over 50% of its vitality, but applied heavily enough, it will be sufficient. On another note. The amaryllis (Amaryllis sp.) and the snowdrop (Galanthus sp.) are only related by family (Amaryllidaceae), and are almost certainly not genetically capable for cross pollination.