Fat soluble vitamins will begin to decline in properly stored hay at about 18 months. Since this decline is gradual, if hay is stored out of the weather and kept dry, the "shelf life" on your rye grass hay should be approximately two years. After two years, nutritional value will decline, and the level of dust in each bale will increase. Be sure your hay has been baled at the appropriate stage and is stored properly.
Here is the link to a fact sheet on hay for horses:
http://www.extension.umn.edu/horse/components/pdfs/FactSheets/HayForHorses_08463.pdf
