Packaged dry chemicals should generally be mixed in their entirety, because you can't guarantee that the components are uniformly distributed in the bag. If you can't use a whole bag of D-76 before it expires, and you can't get a smaller bag, then you might consider mixing your own chemistry from bulk chemicals in whatever quantities you need.

To get started, take a look at Steve Anchell's The Darkroom Cookbook--

If you're asking how long packaged chemicals can keep in their original packaging, usually for a very long time--years.

If you're talking about the storage of bulk chemicals, most of them do just fine on the shelf for years. Some are hygroscopic, so they should be kept dry, and some like glycin are sensitive to light, so they need to be stored in the dark. Refrigeration isn't necessary for most dry chemicals.