200-300 calories a day..how much weight can be lost in a month?

Hey guys, I need to lose weight as I am extremely overweight. So as a start, I want to cut my calorie-intake to no more than 200-300 calories a day. I walk to school and from school everyday so there's a bit of excercise there. As well as using stairs everyday. Tell me, how much roughly can be lost in a month by this diet? And I don't have time for the gym as I'm always studying