You can never get all of the venom out of a snake so even though they have been milked there is still enough in the snakes reserves to help it hunt or protect itself and the venom stores will be fully replenished in a couple weeks or sooner if it has a nice meal. Some rattlesnakes may feed on a large meal, such as a rabbit/piglet/ground squirrel maybe once a year with other small feedings along the way.