The elemental question is whether you even engage the services of a coach, and the answer is that there are no medal winners without coaches.
One client calls every couple of years, and if that's OK with him, then it's OK with me, but I think he could achieve his goals a lot faster with more frequent sessions. Another client calls on a very irregular basis, sometimes a couple of times a month, then not at all for 6-9 months. That's the worst, if you ask me. I think consistency is of the utmost importance. Building a rhythm is essential.
On the other end of the spectrum, I have had clients who could not be helped even if the meetings were weekly, or even daily, because they could not take the actions that were needed. For one reason or another, they just had other priorities than to do what needed to be done (and what they agreed to do) in order to achieve their stated goals.
"Actions express priorities"
The frequency that seems to work best is either bi-weekly or monthly. Monthly works OK for people in slow moving businesses. Bi-weekly (every other week) works best for people in faster moving situations such as startups. Bi-weekly give the client enough time to work on their priorities, make changes, and get feedback, but it's not so long as to get too far off track.
In between bi-weekly meetings there are usually a few emails, documents, or phone calls to ask questions, clarify points, and touch base on urgent developments.
The best results I have found are with one 1-1 coaching session per month, coupled with one board of advisors meeting per month, for a total of two meetings, but in different formats.
The real question is: how fast do you want to achieve your goals? How motivated are you?