Unused regular single day and multi-day tickets don't expire. On multi-day passes you can typically use a few days on one trip and more on another. Unused days don't usually expire unless you have a "2nd day free" pass or such which often gives you 7 days to use the free day. Also, Unlimited Park Hopper type media offered through Disney Resort packages are only valid during the time of travel. Annual passes expire one year after activation (not purchase).
Hope that helps.