Moisture will darken and eventually clump up the powder but it will still work. If it starts to clump, simply crush up the clumps with a DRY utensil (perhaps the scoop that comes with it) and continue to use it.
The powder usually starts out very, very light gray, sometimes almost white. Keep it as dry (stored in a climate controlled environment) as possible.
Over time, it will continue to darken and clump but, again, it will still work. You are looking for this.......if it turns chlorinated water pink, it is still good.
In time, it will clump badly enough you will be tired of messing with it and order a new refill but, controlled as carefully as possible, it should last in excess of a year or so.