Vitamins can last from 6-12 months, depending on the brand and if you store them in a cool dark place away from sunlight. Your tub SHOULD have a use by date at the bottom, go by that.
As for calcium, (powdered form, also human grade) it can last up to five years, in a cool dark place, if it's been in the sunlight, then significantly less. (if mine had been in sunlight prior to finding out, I'd dump it and get some more)
hope this helps