The amount of calories you burn on a treadmill is going to vary based on the incline and resistance settings....The more intense the workout is, the more you're sweating and exerting yourself...the more calories you're likely burning.
If you're at an incline and running at a pace where you're perspiring after 15-20 minutes, chances are you'll burn 500 calories in 1 hour.
If you're relying on cardio on a treadmill alone(not diet) to lose 2kg(4.4lb), you'll have to burn 15,400 calories.
To burn 15,400 calories in a period of 1 week where you burn 500 calories in 1 hour, you'd need to spend 30.8 hours on the treadmill that week (4.4 hours/day).
Something tells me you're not going to want to do that, so I would suggest you aim for a more reasonable weekly goal of .9kg/week(2lb/week).
You're also going to want to count your calories. I wouldn't go lower than 1,200/day as a woman or 1,500/day as a man.
What you should look into is a strategy on how to go about losing weight quickly and easily but in a realistic amount of time.