I like Buffy's walks to add up to about an hour a day. She usually goes out about 4 times a day: morning, lunch, evening, and right before bed. Length depends how much time I have; for example, if I'm running late, the morning walk gets abbreviated. The one right before bed is almost always abbreviated since our place is not great after dark.
Sometimes that doesn't happen, but we also take her out for longer walks usually once on the weekend and once halfway through the week. I also try to do short training sessions with her, as those tire her out as much as walks. I often have her work for breakfast or on her walks. We do the dog beach and dog park quite often too, and she has to work there too.