How many times a week you need to walk to lose belly fat depends on the amount of belly fat you want to lose. Follow a healthy diet along with your walking routine. If you eliminate 500 calories per day, or 3,500 calories per week, you can lose one more pound of belly fat each week.
Plan different routes to walk each day for a change of scenery and to avoid boredom. Add hills, steps or stairs, etc. for a more intense workout and to burn triple the amount of calories. Add variety to your routine.
And, most of all, set realistic goals for yourself and track your progress.