How long should a 66 year old exercise in order to lose weight in a healthy way?

1-2 hr, 3-4 x/wk: Duration, rate, and intensity are the 3 variables you can "titrate to effect" when it comes to tailoring workouts that maximize you desired gain (loss). After being cleared by your md, you can dial up or down the duration of workouts starting with ~20 minutes and building toward 90-120. Intensity decreases as duration increases. Drink plenty of fluids to keep your calorie burning machine efficient.