How long does chocolate last?

We are wanting to do favors with hershey chocolate bars for our wedding Aug. 1, 09. Since we are having over 300 guests we want to keep our costs to a minimum, so I thought buying just after v-day would save us some money, but is it worth it? How much in advance can we prepare these favors without the chocolate going bad or stale? Every website I have visisted has conflicting answers...can anyone speak from experience?