White labs has a best buy date on the vial--4 months after the date of production. I'm not experienced enough to throw out exact numbers, but to my knowledge the older the yeast (especially liquid), the less viable it is and the greater the need to make a starter even if 1 vial would have been enough for the batch. I've also heard if it turns the color of peanut butter, its no good. Of course, it should be stored in the fridge. Dry yeast has a much longer shelf life. For what its worth I just took a look at a pack of s-04 I've got in the fridge, purchased a few months ago, with an expiration date of 7/11 printed on it.
My practice with liquid yeast has been to brew as quickly as possible after receiving it.