How much daily physical activity do i need to maintain my weight at a stable level?

Depends: It changes based on your age, exercise level, diet, and weight. Get at least 150 minutes a week of moderate intense exercise (difficult to sing) and a healthy diet to maintain weight. More important than exercise is physical activity the rest of the day. Stay active every hour by getting up and doing house-work, taking steps, short-walks, squats, lunges, etc. That's what centenarians do.

30 minutes: Current recommendations include 30 minutes of a moderate aerobic activity 5 times a week. If you prefer vigorous activity you can shorten the time. To maintain your current weight you must couple regular physical activity with moderate calorie intake, about 1800-2000 calories per day for the average adult.