Pippin gets bathed every 2 weeks in summer - he is such a "swiffer" dog that he gets quite dirty underneath, and it needs shampoo to get it out properly. Maddie gets bathed every week in summer - vet advice to wash the pollens from her coat. Their bedding gets washed at least once a week in summer as well for the same reason. I use a lovely mild oatmeal shampoo and conditioner.
In winter they tend to wear jumpers when out, so I just rinse their paws off as and when needed, and they only get a full bath if they look dirty or if they are smelly (or when Maddie rolls in something :yuk: )