After seeing my current therapist once a week for about a year, I increased to twice a week. It was his suggestion, but the actual decision was completely up to me. I found it immensely helpful to go more often... It became much more possible to talk about things in more depth, and to feel supported enough to talk about more difficult topics. At once a week therapy felt a little like treading water (or in worse moments, like bailing out a leaky boat). At twice a week it started to feel like we were actually making progress.

I did twice a week for a year and a half, before increasing to three times a week recently. My opinion is that three times a week is great for me and what I need and want from therapy, but wouldn't be ideal for some. But I think twice a week would be a beneficial option for many, many people, and to be honest I think a lot of people would be better off if twice weekly were more of the "default" frequency, instead of once weekly.