In the warm months I wouldn't keep seed longer than a month. Just as a box of crackers kept in a hot garage will go stale (even if the bag isn't opened), seed also goes bad. And while we'll survive the occasional dry cracker, birds have high basal metabolic rates & so use energy at high rates. They don't have time to waste on seed without a high oil content.
Also after seed is harvested, it’s brought to mills where it is processed and stored properly. Along with the seed come eggs that insects have already laid in the field. Birds don't mind the extra protein, but if the eggs hatch they can cause a real mess.
Indian meal moths or pantry moths lay tiny eggs (0.5 mm) in or near potential food plants. When conditions are right, larvae or "tiny whitish caterpillars" hatch, eat, and then spin a tiny silken cocoon.
Tips to keep your bird seed its best:
2) Storage Conditions - Store seed in a cool, dry place, or a freezer to keep seed fresh and prevent moths from hatching. In the winter, the bird seed can be stored up to 3-4 months in a steel can outdoors or in an unheated garage.
3) Sterilize Seed – You can use heat to kill any larvae and eggs that may be in the food. In a conventional oven, spread seed out on a flat baking sheet and cook for 30 minutes at 300 degrees.
4) Seed Selection – You are less likely to find eggs in seed that has no shell. When sunflowers seeds have their shells removed it also removes almost all eggs. Wild Birds Unlimited No-Mess Blend or Shelled Sunflower seeds rarely carry eggs, but may be re-infested if Indian meal moths are in the home. Nyjer (thistle) and safflower seed are also less likely to be infected initially.
What is No-Mess Bird Seed? http://bit.ly/tRaefu
How long does bird seed stay fresh? http://bit.ly/rTLSqJ
Seed Storage Cans and WBU Seed Scoops http://bit.ly/uBaSwO
Sunflowers Up-close: The Strange Journey of an American Plant http://bit.ly/uFlz65