How long would it take on a proper diet to put on weight?

Im not saying how long do i have to be on a diet for

Im saying im underweight, and i only eat 1 dinner a day that my mum cooks for me. when i start my new job if i pay for breakfast lunch and dinner everyday and drank 2L of water everyday and ate my 5 aday everyday and did it all, would it take long to put on enough weight to be a healthy size? if not ill try eat 4-5 meals a day everyday

Thanks