O rings are made of a synthetic rubber Nitrile material, whose properties change over time. Cared for, and kept in a controlled environment, bands ought to last 2 years.
Care:
Photosensitivity - Bands will become brittle when exposed to sunlight. Remove them before leaving the Unapper in the sun for your bees to clean.
Chemistry -  some oils or solvents will dissolve nitriles. Washing by hand in water, and drying, will preserve their integrity. 
Time under tension - Hysteresis and extended time in tension will affect the elasticity. Remove bands between uses and for best results we recommend changing them each year.
 
You will get 12 bands with every Uncapper so only order these replacement packs if you have worn through your entire stock.