Shelf life could be forever when they are in their package, sprayed with a proper conservation grease, if temperature and humidity conditions do not vary too much, the bearings are not exposed to vibrations in the storage room and the bearings are turned every six months.
There is no set answer to this question. It depends on the environment that they are stored in. The biggest problem would be vibration. If you are in an industrial plant there is probably enough vibration to cause a problem over time. In most applications this won't hurt you unless the bearings have been stored forever. For precision bearings this could become a big problem in a very short time.