Hi Julz, and welcome to the forum. 100g a day loss for me is realistic, so that is about 260 days plus 20 days because with 5kg or less to go, I'd lose about 70g a day. So that's 280 days, which is September 2nd next year according to Wolfram Alpha.
As for energy use, enter your details into this calculator http://www.bmi-calculator.net/bmr-calculator/
which is the energy needed to keep your body working. If you're 6 ft, enter 5 ft 12 inches. This gives me 1883 Calouries, which is 7880 kj.
To determine your total daily calorie needs, multiply your BMR by the appropriate activity factor, as follows:
If you are sedentary (little or no exercise) : Calorie-Calculation = BMR x 1.2
If you are lightly active (light exercise/sports 1-3 days/week) : Calorie-Calculation = BMR x 1.375
If you are moderatetely active (moderate exercise/sports 3-5 days/week) : Calorie-Calculation = BMR x 1.55
If you are very active (hard exercise/sports 6-7 days a week) : Calorie-Calculation = BMR x 1.725
If you are extra active (very hard exercise/sports & physical job or 2x training) : Calorie-Calculation = BMR x 1.9
So that is 7880 x 1.2 = 9500kj. If I want to lose 100g a day (3500 kj) I need to eat 6000kj.