Depends on if it's dry yeast or not. Both should do just fine for quite a while if they are refrigerated. The dry yeast, while not the preferred yeast of many, will last a very long time if kept dry and cool. The Wyeast Activator packs should also last a while under the same conditions.
The only rumblings I hear around here about ordering yeast is about doing it in the summer when it's very hot outside and not being able to know how the yeast was handled in transit.