#175--if the goal of your workout is body transformation then it needs to be changed frequently. The body will adapt to any stimulus. And if that stimulus is cyclical in nature (i.e. cardio), then that adaptation occurs very quickly. Paradoxically enough, from a body transformation perspective, the better you get at a particular movement/exercise the less effective that particular movement/exercise is for you.
For example, I've ridden bicycles for over half my life now. I used to compete at a fairly high level, and I have thousands of miles in my legs. Thus, if you and I go out and ride the exact same course at the exact same speed and under the exact same conditions (i.e. we ride the same type of bike, we weigh the same, etc), it's safe to say I'm going to burn a lot fewer calories than you. And from a performance standpoint, that's a good thing. I race. I need to be efficient. I want to be able to spend the minimal amount of energy necessary to complete a given distance at a specified speed so I can have more "left in the tank" for the final sprint or for the 26.2m run I'm doing off the bike.
You don't want to be efficient if your goal is to change your body. You want an exercise program that keeps your neuromuscular system challenged so that it's constantly trying to adapt. You want a program that is periodized such that not only is the training focus continually changing, but the actual movement being developed is changing, as well. Weight training is the key, allowing you to manipulate variables such as intensity, reps, sets, rest intervals, frequency, and the exercise(s) performed.