It depends highly on how fresh the vanilla is when you buy it. At least here in Germany, vanilla beans from the most common super market brands are mostly parched and IMHO already out of order when you pick them up in the store. For several years, I have ordered my vanilla in larger batches from an independent importer (current price is about 30US$ for 60 beans) and I've at least kept them for a year and a half or perhaps two. The quality of course deteriorates with time, but they are not going bad as in "cannot be used anymore".
One problem is however that if the beans are too fresh, they are a little bit tricky to store properly. As scientifics already wrote, it is usually recommended to keep vanilla in an air tight container to prevent them to dry out. If the beans are very fresh and still moist, this will cause water to condensate and I suppose it is not good for the beans to soak in water for a longer period of time.