Well, you can never over do it. In fact, as often as you can will keep your bases running fast and more durable than a non-waxed base. If you wait until your base looks white, like dry skin, then you have gone too long and need to get a base grind to remove the dry, dead layer of base.
If you do it once a week, you will be well ahead of most anyone. Again, you can never wax too often.