Unless they bought a single 20 lbs bag, or they open all four bags, they have 15 lbs stored airtight and dark by default.

Also, I hear kids these days occasionally use the Internets to learn how to do things.

But even if you open them all and let them sit on a counter, you get at least several months out of them unless you get insects in there. And if that happens, trust me, you've learned a life-long lesson.