How many calories do you have to eat to maintain a weight of 124 lbs?

Assuming you are Female and 35 your BMR is 1328.8.

That means your body burns 1328 calories if you were to do nothing.

Lets say with additional activity (work, exercise, walking etc) you burn another 500 Calories, that will bring you up to 1828 calories per day.

Then you eat a 1500 calories diet (pretty normal, just need to watch what you eat), you will have an excess burn of 328 Calories per Day

It takes 3500 calories of work to burn 1lb of fat

So you will need to burn 24,500 Calories in Excess of what you consume.

24,500 / 328 = 74.695 or for simplicity sake, 2 and a half months.