The short answer: less often than people think. A healthy dog with no smell and no dirt needs a bath every one to three months. Everything else is solved with plain water.
Why not human shampoo
Human skin is acidic; a dog's is closer to neutral. Shampoo made for us is harsher on a dog: it strips the oily film that holds moisture and keeps bacteria out. Once will do no harm; regularly you get dry skin, dandruff and itching, followed by scratching sores.
The same goes for baby shampoo and soap: "gentle" does not mean "right pH".
How often, really
Short-coated dogs without an undercoat — every one to two months. Long-coated dogs and those with a dense undercoat — every month or six weeks, but always brushed out before the bath: a matted undercoat gets wet rather than clean and then takes a day to dry.
Dogs with skin conditions follow the schedule their vet sets; there is no general rule there.
Between baths, rinsing the paws and belly with warm water is enough. City dirt washes off with water perfectly well.
How to do it
Brush before the water, not after: a wet mat tightens permanently. Water should be warm, barely above room temperature — hot water dries skin more than any shampoo. Dilute the shampoo in water rather than pouring it onto the coat: concentrate rinses out poorly and stays behind to irritate.
Rinse for longer than feels necessary. Shampoo left in the coat is the most common reason a dog itches "after grooming".
Keep water out of the ears. Wash the face with a wet hand rather than pouring.
Towel first, then a dryer on warm — not hot — if the dog tolerates it. A dense undercoat left to dry on its own stays damp underneath, and that is where the smell comes from that makes people bathe more often, closing the circle.
Winter in the city
De-icing salt from the roads is its own matter: rinse the paws after every walk, with plain water. Not because it is dirty, but because the dog will lick its paws at home.
When bathing will not help
A persistent smell on a clean dog, constant itching, bald patches, red skin, dandruff that does not clear — none of that is about bathing frequency. Bathing more often in that situation makes it worse. That is a reason to see a vet: the causes vary, and none of them is solved by shampoo.