This calculator will estimate the time you need to reach your weight loss goals at a rate of 1-2 pounds per week (about 0.5 - 1kg per week).
Healthy weight loss happens at a rate of 1-2 (about 0.5 - 1kg per week) pounds per week. The reason why it's a bad idea to lose weight faster is because more weight loss requires harder change. Losing 1-2 pounds per week is already a difficult task. You need to cut calories and increase your physical activity level. If you're not used to this sort of lifestyle, it will put both a physical and mental strain on your body.
If you try and lose weight faster than the recommended rate, the changes that you will need to take will be too much for you to handle. After a few weeks of being on an extreme diet and exercise routine, you'll quit because you simply can't handle the new lifestyle. You'll soon return to your old ways of eating unhealthy and exercising very little.
The best approach is to change slowly which will allow your body to better adapt to your new lifestyle. Slowly add more exercise into your daily routine and carefully replace unhealthy food choices with healthy ones. This will give you the best shot at successfully keeping the weight off.