If you keep pallets out of the weather they last nearly forever especially if you can get the really good heavy-duty pallets like they use for moving cement or concrete mix. Maybe you can get some of the good heavy oak ones from Lowes/Home Depot but it may cost you some because pallets can be returned for a refund though I think that there aren't many people (not businesses) who do return them. Everyone needs a pallet or 2.

You might also want to lay some gravel before you put the pallets down so that they don't sink into the dirt with the weight of the firewood and the grass and weeds that may grow up around them. It would keep them in better condition for longer.

Recycle, reuse, and reduce the waste then build to last!