Unfortunately, the answer to this question is "it depends." Certain factors like genetics, how much weight you have to lose, and how close you follow the diet play a big factor. However, I think we can safely say that most people start noticing major changes at the end of phase II and beginning of phase III. This could be because the change in workouts during phase II help break the plateau; it could also be that you are working out with better form after getting used to the first couple of weeks, and upping reps and stamina. Another factor is that the stress your body is undergoing in the first weeks might cause you to retain more water, which will keep the scale number higher than you want. Either way, no matter the explanation, stick with it and you'll see your results soon!
Of the 3 factors I list, following the diet is the only one you can actively control. Eating clean and within your caloric range will probably do the trick as well, but I often tell people at least the first time doing p90x, you should follow the diet plan as closely as possible. There is a lot of thought that went into the plan which is tailored for the p90x progression. Doing the workouts, you will feel better and get into better cardiovascular shape, but if you want the look, diet is key. Feed your muscles what they need for building, and avoid the fatty or sugary or high-salt foods that are so easy to eat, but will hinder your fat loss. To answer your questions about the meal plan, go here.
How to measure your results
Often, the scale isn't the best way to gauge results, though it's so tempting to rely on the scale for answers. First of all, our body weight shifts in and out throughout the day. Second of all, you will likely be building muscle, which is denser than fat and therefore less volume of muscle weighs more than the same volume of fat. If you measure your body fat %, this is probably the best way to look at your results. Body fat % is not the same thing as BMI (body mass index). BMI is not a very important statistic, it basically takes a ratio of your height and weight, and determines if you fit in the normal range or not. To estimate body fat percentage, log onto teambeachbody.com and click on Eat Smart > Nutrition Tools and there is a body fat calculator there. It is basically an estimator using your weight and size of your waist. Click here if you need a team free team beachbody account. You can probably find other body fat % formulas online too if you google it. You'll be surprised if you measure inches lost how much a single inch can affect your body fat %!
Finally, the best way to determine your results is to measure how you look, how you feel, and how your clothes fit! We all want to quantify everything in our lives, but in reality if you go by feel you are going to be just fine. P90x is about more than just a body transformation, it's a mental transformation as well and breeds a healthier lifestyle!