As long as the question is how often COULD you bathe your cat, then I'd say as often as every 2 weeks. As long as it is a shampoo made for cats and ALL the shampoo is rinsed out and they don't catch a chill (proper drying). The act of bathing in itself would not cause skin problems. Shampoo left in the coat might, or an allergic reaction to the shampoo might, but not the actual bath. If the cat has any type of skin problem to begin with you'd want to choice your shampoo and frequency with care, though.
Consider this: Cats get into the same pollutants that we get into (dust, anything airborne, and dirt if they go outside, etc) and we wash it out our OUR hair. Just because the cat licks himself is not a just reason to not bathe them with soap and water. The more "stuff" they have on their fur, the more "stuff" they will injest.