it lasts a lot longer if you can mix as you develop. powders like d-76 have to be mixed all at once so they don't last as long, though i've used mixed d-76 after a year. many liquid base developers will last a long, long time unmixed in an airtight container.
some can suffer from instant death, like diafine. it can last a few years but will suddenly not work at any given point in time. color developers suffer from instant death as well, sometimes.