A reasonable starting point
For a healthy dog with a normal coat, somewhere between every four and eight weeks suits most. Short, smooth coats often go longer. Long or oily coats, and dogs that spend a lot of time outdoors, tend to sit at the shorter end. Puppies and older dogs generally need fewer baths, not more.
Signs you are washing too often
A coat that looks dull or feels dry, flaking at the skin, and scratching that appears a day or two after each bath. Frequent washing removes the natural oils that keep the coat supple and give the skin time to recover. If any of these appear, lengthen the interval before changing anything else.
Signs you are not washing often enough
A persistent doggy smell that returns within days, a greasy feel to the coat, dirt that brushing no longer lifts, or skin folds that stay damp. Some breeds simply produce more oil and need a shorter interval.
Between baths
Most of what people wash for can be handled without water. Brushing removes loose hair and dust and spreads natural oils along the coat. Wipes handle muddy legs, paws and faces. A damp cloth handles a single dirty patch. Reserve the full bath for when the whole dog needs it.
When the interval is not the problem
If a dog smells strongly of yeast, has red or broken skin, bald patches or scratches enough to disturb its sleep, no bathing schedule will fix it. Those are reasons to see a vet, and washing more will usually make them worse.