How long does bicarbonate soda (baking soda) keep? I am thinking of baking biscuits for presents for Christmas.

However, the recipe calls for baking soda, and I cook from scratch fairly rarely, so I'm worried the leftover bicarb will go strange in my cupboard.  And since people use it for all sorts of purposes, it's one of those products generally not available in the small quantities called for in cooking.

If I do buy a packet, how long is an expected shelf-life for it?  Based on this question I probably should find an airtight container to store it in, which may assist.

(Note if relevant: I live in Sydney Australia; Christmas here is mid-summer, and often fairly high-humidity and 35-40 degree C heat.)
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.