They are inedible and inert to begin with, so their shelf life should last into the next century and still be safe. Lemony flavored cardboard I say. And try to have more water on hand than your ration may allow too. Its a slow gnaw without something to wash it down.

Last summer I tried making bread by grinding up a Datrex and using it as 1/3 of the flour in a loaf of baked wheat bread. It made a lemony lumpish loaf that didn't quite rise enough. With some trial and error I might get the proportions right to make lemon cake. The intent was probably pretty close to what folks have done in times of no food, when they add sawdust to a bit or remaining flour. When I'm bored enough to try again I might chip off some more from the Datrex block and try again.