I keep my unused pellets either in a Kingsford charcoal container (holds 40 pounds, water-tight lid, handle to help lug it around) or in sealed 5 gallon buckets from Homer Depot. Small amounts go in commercial Folgers coffee containers. Most of them are a mix of this and that..
I think that depends on where you live, weather conditions and time of year. Believe me you'll be able to tell.
Just last evening I had problems with some that were left in the hopper. With all the rain we've had lately they'd picked up enough humidity that they were hard to light. I removed & mixed them with some dry ones, refilled the hopper and they were fine.