3 minutes without air.
3 days without water.
3 weeks without food.
But losing weight, you can't starve yourself. Your body will go into shock and save energy, hence you won't lose weight if you don't eat.
Thats not completely true, you would begin "eating" yourself, decomposing less important (than brain and his support organs) organs to gain energy for brain, untill you would eat them too and let brain to be last fully functional organ. At that tme you would also be most likely unconscious and on the edge of living. But this will happen when you don't eat in long period of time, close to that 3 weeks.
To the topic: I have really bad eating habits, sometimes I ate everything i can reach for few hours and then there are times i go without eating anything for more than 24 hrs. Luckilly i drink about 3 litres of water a day, at least something i am doing about right