How long will it take for me, a 13 year old boy, to get a tummy thats flat from hip to hip and a six pack?

as the title says. i am a 13 year old boy who desperately wants the above, and i would like to know how long you think it should take. i am 5 foot 2 and weigh about 45 kg. i have consulted a doctor and i am going to start going to the gym 3 times a week at twenty minutes a time and hope that this will get me what i want (also, if you have any practical advice or help in this regard then that would be great). i dont want to get bigger and more hunky, i just want to lose most of the fat that i have (although i want to put on the equivalent weight that i lose in fat in muscle, as i really dont want to actually lose weight on the scales). yeah, just to clarify, i dont want to get bigger in any way, i want to get smaller, so that means getting a six pack by getting rid of all my fat rather than making my abs bigger. cheers in advance