They rot within 3-4 days of carving, so you'll want to hold off until just a few days before Halloween...unless of course, you want to do more than one batch of Jack-o-lanterns this year.
This is what we did last year.
You can extend the life of the carved pumpkin by rubbing Vaseline on the carved flesh and the insides, thus keeping oxygen from getting to it and slowing the rate of decay.