How often do you bathe your dog?

We have a dog, mixed breed, medium to large size, medium length coat. We live in a flat and we wash his paws everytime after taking him for a walk, and he gets a full shower when he gets really dirty somewhere in the mud, rain etc. Which is usually once a year. But apart from that we don't do it since my boyfriend insists that dogs shouldn't be bathed often. However we have friends who bathe their dogs monthly. I never had a dog prior to this so I really don't know what is the best for them. Any advices, experiences, thoughts are welcome!