Usual advice is it will last as long as you don't open it and don't leave it to get wet but I've used last years opened compost and not noticed any difference. More important is the brand - if its a good brand you shouldn't have a problem. If its the cheap stuff to start with its not a lot of use for raising young plants or seedlings.
Some folk will probably disagree though.

It'll be fine -if you open it, just make sure you close the bag afterwards by rolling the top down and standing something on it. I've always got a bag of Wickes multi purpose on the go in my garage, and it might be there a year. Don't store it outdoors over winter if you can avoid it though

If it's cheap then you can always use it for baskets and containers for summer displays if you don't open it. If you intend to sow seeds in the spring then buy the best seed and cuttings compost. It can be pot luck as far as cheap compost goes; sometimes full of what looks like floor sweepings. One year I bought Miraclegrow compost and although it was what I considered top of the range it was full of rubbish. Recently I bought B and Q Verve Cutting and Seed compost and it was excellent stuff.

I usually tell my customers that it is good as long as you keep it dry. A minority of the soil organisms in it will die, but the rest will retreat to spore or egg form, and revive once they get moisture and room.