I've learned the hard way that Grain looses it's freshness after about 12 months and becomes stale tasting after 24 months. (I took a break from brewing for 2 years, and started back using my existing grain stock. Couldn't figure out why the first few batches tasted so...meh, but figured out eventually that it was the old grain.)

Grain stales quicker when left exposed to air (oxygen) and moisture. If you think you'll use the grain within 6-12 months, then a trash can is fine. Longer than that, and you should probably consider something that its airtight. To keep grain in the freshest possible condition, use an airtight bucket and some oxygen absorbers. 

For speciality grains, which are typically used less and hang around longer than base malt, I use a foodsaver and package in 0.5lb and 1lb quantities. 

See also

- [Airtight gamma seal lids and buckets][1] 


  [1]: https://homebrew.stackexchange.com/questions/6289/gamma-seal-lids-and-b3-buckets