If the compost is dry and has been kept dry then it will last indefinitely. If it has been allowed to get wet, and many Garden Centers still keep compost out in the open, then the nutrients will have started to react and break down and it won't last for long, maybe a year. You could still use it later with a bit of added fertiliser but the amount you add will be pure guesswork.

Bulbaholic's right - but it depends on what kind of compost it is. If its seed and compost or ordinary multi purpose compost, once its wet, you can reckon any nutrients added to it will be gone within 2-6 months, so add a handful or two of Growmore or fish blood and bone when you do use it, depending on the size of the container.

Many thanks guys. It's a compost that the local nursery makes up and it seems much better quality than anything from large stores. I want to use it for my seeds in Feb / March .

Then don't add fish blood and bone or growmore, if its for seeds, Pdb. Just store it somewhere dry.

The only compost i'd have doubts about storing for any length of time are those that have controlled release fertiliser in them. Apart from that, as the others have said.

With regards to your compost, it sounds too good for seeds. I'd just sieve any of the large stores "seed compost" to get rid of the rubbish and use that. For the few weeks that seeds are in compost before being transplanted it seems a shame to use up good stuff for that purpose.