How long does bicarbonate soda (baking soda) keep?
StillTasty provides a very conservative answer: 6 months at best quality. In my personal experience, it can last much longer and be just fine, though I think the longest I ever kept a box was a couple years, not 5 years like riotburn. StillTasty suggests that the concern after more than 6 months is lost potency, but I believe the potential to take on odors from the surroundings is much more of a concern; store it in something airtight like you said, and you should be pretty safe from that.

So I'm sure it'll last at least until next Christmas. And if you do get into baking, you might go through a box faster than you'd think. Otherwise, it's fortunately pretty cheap, and it does have other uses (e.g. cleaning) so it won't hurt you too much to just go for it.