Scout has never had an ear infection but her ears seem to get dirty pretty quickly. I clean them at least once a week and every time she goes swimming, which in the summertime is every weekend. I can't believe she's never gotten an infection considering how much time she spends in the water.
I voted bi-weekly, because that is how often I usually end up doing them. They get baths "as-needed" but at least every month, and always have their ears cleaned after. They also have their ears cleaned anytime they get wet.
Every day or two I wipe the ear out with a baby wipe, and weekly I use the ear cleaner for a deep cleaning. All of my dogs have clean, fresh smelling ears, and none of them have ever had an ear infection.
I clean Merlin's ears as needed. While I check them (includes smelling them ) often, they stay really clean. I think he's had one ear infection in his 2.5 years. I feel lucky that he is not prone to them.