last year, most will sprout unless they got wet then sometimes even then, I have had many year old seed come up <img alt="" class="inlineimg" src="http://www.mothering.com/discussions/images/smilies/smile.gif" style="border:0px solid;" title="smile"> If you are in doubt can try sprouting them between 2 damp paper towels in a ziplock bag, sometimes you will want to plant closer or a couple seeds per spot where with new seeds you would only plant 1 per hole.
I've read about people putting thier seeds in the freezer and them lasting 20 yrs. Not all sprouted, but some did. I have seeds that I'm planting from like 4 or 5 yrs ago. Not all sprouted, but quite a bit did. They say seeds can keep from about 3-7 yrs depending on the type of seed.