There's no single answer, but there's a good rule of thumb: bathe when it helps, not on a fixed calendar.
Short-coat, indoor dogs: once every 4–6 weeks. Their natural oils do most of the work.
Long-coat or double-coat breeds: every 3–4 weeks with proper drying, otherwise mats and hot-spots creep in.
Active outdoor dogs in monsoon: rinse-only baths in between, full shampoo baths every 2–3 weeks.
Always use a dog-specific shampoo. Human shampoo — even baby shampoo — has the wrong pH for canine skin.
