Waxing depends on how the old wax is doing. Most usually need a top up every four or five weeks. You can tell by the beading - if it starts to flatten a bit, it's time for topping up. Provided it's the same wax, a good wash, dry and then can just apply a single coat of wax. Don't need to remove old wax unless you're polishing/applying a new sealant/wax.