I get in about 30-40 minutes 3-4 times a week. So far that is enough for me to help in my weight loss. I imagine that as I get fitter and thinner, it will take me much more to lose.
My friend has been on WW and exercising for 2 years. It used to be that a 2-3 mile walk was good enough for her weight loss. Now she has to put in about 7 miles to see any effect. Yikes.