With the weather the way its is, ours get bathed about once a fortnight but it has been once a week sometimes. Depends where they have been.
When we picked them up from kennels last time, they had been bathed the night before, then we took them out for a really good run, they got really wet and very muddy, so another bath was needed. THis doesn't happen very often.
I have just bought shampoo from crufts, one for black coats and once for white (Solid blac and orange roan cockers!).