This can be different for each individual. However, it is generally recommended that you lose no more than 1-2 pounds per week. To achieve this, a reduction in about 500-1000 calories per day is needed either from decreasing calories consumed, and/or increasing calories used with activity.
To estimate how many weeks it will take you to achieve your weight loss goal, divide the total number of pounds you'd like to lose by the rate at which you are planning on losing them (e.g. 1-2 pounds per week). If you are already eating relatively few calories or are at a relatively healthy weight, you'll want to make smaller reductions to your calories, and expect to lose weight more gradually.