How many hours a day do I have to be at the gym to lose 2.5 pounds a week?

I have a goal set for myself to lose 40lbs over the next 4 months. It's about 10/lbs a month, 2.5lbs a week. I would like to know how many hours do I have to spend at the gym to lose that much a week? I'll be highly committed and I will be watching my diet. Plus I'll be keeping myself pretty busy to even be eating any junk food. I gained all this weight during school. Also how many calories a day would I have to intake/burn?

Well its 3000 calories to a pound, you can burn 1200 each hour, well i know i can.