I go every week.
Therapy is so crucial for me that I drive 2.5 hours and into another state to meet with my therapist. I had been living in the same city. Then I moved and tried a different therapist, and it went really badly, so I started commuting back. He isn't working on our usual day next week due to the holiday, so I'm going to have 10 days between sessions. I'm already stressing from it.

Like BeyondTheRainbow said, I need someone around when I'm low or in an episode because I can't think clearly. I tell him the thoughts and feelings in my head, and he tells me a different way to think about it, and we make a plan for how I'm going to cope that coming week.

Once a week works perfectly for us. Longer than a week, I'm bursting with stuff to talk about. If we were to meet more often than weekly, I think I'd run out of things to say.