Visa can go months and months and still smell like shampoo, but I bathe her about once a month.
Mr Tandem on the other hand is the stinkiest dog I've ever had -- even though Visa can participate in all of the same activities (swimming, dog park, etc), Tandem smells like an outside dog two days after a bath. Therefore he gets it about once a week or two. Maybe it's because he's almost a teenager. ;)
And -- a good quality shampoo should always put more in than it takes out. So as long as someone buys a really decent shampoo, they shouldn't worry about the skin drying out. Since I'm a groomer I order many different kinds of great shampoos, and I never have trouble with dry skin.