As often as you feel comfortable. Zuzka recently answered this in a Tweet:
@eatabanananow 3-5 days!
— Zuzka Light (@ZuzkaLight) January 9, 2013
My Personal Experience:
Currently I do one a day Monday-Friday and rest on the weekends but that changes depending on how I feel. If my body needs to rest I move my rest days around.
When I first started following Zuzka with BodyRock I couldn’t keep up and I got very discouraged. But, the more I did, the faster I got. When Zuzka started the warrior workouts and released one each week that was all I did, one every week. Then I started building up to one ZWOW a day. Over the summer when the weather was great for working out outside I started doing two ZWOW’s a day. It all depends on your fitness level and your availability.