Do I need to rewax my dreads after every washing? - Dreadlocks FAQ's
During the first month you'll want to keep some wax in your dreads the whole time. You always want to be sure that you don't rewax while you can still feel wax on them. This will lead to over waxing. A good rule to follow is that if you can palm roll a dread without feeling in waxiness in or on it, it's fine to add more.
Of course as your dreads mature they no longer benefit from having wax in them. For a more detailed explanation go to the question "How much wax should I use on new, medium and mature dreads?"