If you want the long-term keeping, Multigrade will be a better option. It keeps for a few years as concentrate and will last for a session once diluted.

Dektol will keep forever as powder, but once you mix it up to a stock solution, it is meant to last only a couple months. You then dilute the stock to get the working solution, which lasts a session or so. However, I suspect that by keeping the stock solution in a mylar bag-in-box, you might make it last a year or more.