How many days to lose 15 kilograms in body weight?

Step 1: Figure out about how many calories you need per day. This will depend on your Basal Metabolic Rate (BMR) and daily tasks. It's very important to understand BMR! It's your metabolism and it's based on your weight. Here's the important bits:

BMR is calculated based on your weight.

Muscle burns many more calories AT REST than fat.

Most BMR calculators have no way of imputing your fat/muscle ratio, so what you get is an average.

BMR is what you would burn if you lay in bed all day. It is not enough calories to maintain weight if you are an active person.

Now figure out how many calories you will losing via exercise.

Both things can be calculated online fairly easily.

15 lbs = ~52,500 calories.

So, for instance if you require 2,000 calories to maintain weight, if you only consume 1,500 calories a day (this being the cumulation of both your food choices of the day and exercise), then it will take 105 days to lose the weight. I know this seems like a long time, but losing weight super fast will only cause your body to adapt and make things ever harder! It will go into survival mode which will cause you to feel terrible, tired, and your body will hold onto all of it's energy storage (aka fat). So take it slow, be kind to yourself. This is an investment in your body. Instant gratification isn't worth the suffering down the road.

But beware! As you lose weight, you BMR will change. You will begin to need fewer calories as you lose fat which contributes to your BMR. A way to battle this, is to exercise of course. But you see, as you exercise at the beginning, you will build up muscle, and you will be able to take more strenuous activities more easily. The combination of the muscle (which burns more at rest, remember?), and exercise (which also increases BMR) will help.

Of course, you are also going to gain weight in muscle mass! Muscle weighs more than fat, so don't agonize at the scale, take pictures and look in the mirror to notice differences!

If you start and make an effort, you should notice changes within the first few weeks, not matter your weight. The lighter you get, the harder it becomes to drop a pound. Just don't forget what you've achieved! And be damn proud. Changing habits and forcing yourself to not eat that one thing, or to get on the bike and ride for thirty minutes is NOT EASY. And you're awesome for being able to do it! Never forget that.