Many variables involved with this one - temperature, sunlight, whether they're still on the wheels, stacked or upright, typre pressure, whether they are new or used, etc. If you are just storing over the winter then I wouldn't worry too much. Look here for guidance:
http://www.michelin.co.uk/michelinuk/en/car-4x4-van/storing-tyres-winter/20070319171415.htmlRubber deteriorates with time through exposure to UV sunlight and ozone, oxidation, etc and over long periods tyres will harden and can become distorted by the weight of the wheels etc. There's a bit of info here:
http://www.toyojapan.com/tires/pdf/TTT_07.pdfI remember reading about a 6 year regulated blanket maximum on shelf life for new tyres in the US. However, I don't know whether the EU has enforced legislation. Store them right and they'll be fine for a couple of years.
I store my summer or winter tyres bolted to a frame so they are upright and unloaded and covered up. But I don't think I've ever had a set of tyres for longer than 4 or 5 years.