Gavroche gets bathed every month or two, sometimes less. He's a pretty clean dog, with his hair stuff just falls right off. Even when my nephew and his sister color him with chalk, it's gone within a couple days with no washing.
Logan gets bathed about every other week, more often if he manages to get super dirty, is shedding, or is super flaky. Since he's a service dog he needs to be kept immaculately clean. Over the last month he's been bathed more than that due to preparing for a show. Then the weekend of the show his white parts, particularly his legs, get scrubbed with whitening shampoo daily, plus a rinse or conditioner-only bath on the rest of his body each day so I can blow dry him properly.
I use some horse whitening shampoo on all of Gavroche and Logan's white parts. I use Head and Shoulders occasionally on Logan since he tends to get flaky (and it makes him SO SOFT AND SMOOTH <3). Other than that, it's really whatever shampoo strikes my fancy at the time.