Question about losing weight and a goal?

For the past 6 months my leg has been in a cast and i get it off july 8th!!! (thank god).. But i have gained significant weight.I am 20yrs old Currently I weigh 255 and I am 5,11. I plan on leaving for basic training in January for the Army so i have to lose Alto of weight. Realistically, how much could I lose in 6 months with real hard work? Any tips are welcome