Actually, Wyeast has a 6 month guarantee, from their website: "The Activator™ package was designed with superior UV light- and oxygen-barrier material to extend shelf life, making our 6 month from manufacture date Product Warranty possible."
White labs states 4 months, from their website: "The shelf life for White Labs Yeast is four months. Yeast used after this point is usually fine, but lag times will be longer. There will be living yeast in most vials for 6-12 months, so if a starter is made to activate the yeast, successful fermentations can be carried out with aged yeast."
Both companies state that the yeast is fine after 6 months, but a starter should be made if using liquid yeast that old.