According to Phil, you can double your endurance in just two weeks, but you can also lose it pretty quickly.
The beauty of Peak 8 exercises is that you don't have to worry about the regular, traditional cardio exercises because you're going to get that (and more) anyway through this program. In fact, Peak 8 type exercises can dramatically improve your cardiovascular fitness and fat-burning capabilities in a fraction of the time.
http://fitness.mercola.com/sites/fitness/archive/2010/11/13/phil-campbell-on-peak-8-exercises.aspx