Frequency of appointments with Ph.D. supervisor

The frequency that students and advisors meet depends a lot on the particular student and advisor, and also varies highly by time. In my experience, once a month is quite a bit on the low side, but not unusually so. A more typical pacing is once a week or once every two weeks.

The reason for this frequency is not for the sake of "norms", but rather is a good spacing for giving enough time for the student to accomplish something but not so much so that the student is likely to end up wasting a lot of time or becoming very frustrated going down a bad path that the advisor might have been able to detect and help remedy.

If your advisor is meeting with you once a month and happy with your progress, then perhaps you are simply doing very well at self-direction. It is quite reasonable, however, to ask for more frequent supervision if you think that you will benefit, and as a new Ph.D. student, I think that you likely should do so.

Let me give you an answer from the "high side", just to remind you that Academia varies more than you think it does.

I typically meet my PhD students from three times a week to once a day, at least for a short meeting (or just for a coffee or lunch). Among the many reasons for such high-frequency meetings, I wish to highlight the following:

1. I see here on Academia.SE many PhD students asking questions, like yours, which really should be addressed to their own advisors. So, I make clear with my PhD students that questions pertaining our relationship and their activities should be first addressed to me, whenever the need arises, because it's my duty, as advisor, to answer them. Should I eschew from answering this kind of questions, they'd better call me "unadvisor".
2. There are fields, especially among experimental sciences, where a PhD student cannot work independently on a research topic, but he or she are instead part of a group which works on a certain number of already ongoing experiments. Frequently, these experiments should follow a certain schedule, especially when devices and instruments should be built by other co-workers, or when there is an ongoing international collaboration (e.g. devices and instruments might need to be exchanged between institutes at certain dates). Therefore, I want to be sure that PhD students can complete the assigned tasks on time, and correctly.
3. In certain experiments, mistakes are always around the corner and results need to be cross-checked between different people, even between senior researchers. Therefore, I want to cross-check the results from my PhD students as soon as the data are available.
4. Personal issues can lower scientific productivity (see e.g. How to handle pressure from collaborators when life circumstances make it impossible to get work done?): I think these kind of issues should be discussed as soon as possible with the advisor, and I ask my students to do so.

I think the existing answers are quite comprehensive, but I want to reiterate that you should be asking your advisor what is expected. What I'd like to add is another example of how the relationship can vary.

With my advisor, we would often meet informally several times a day. This was often while critical experiments were happening, or just to bounce ideas off of each other. As my advisor requested for everyone, our lab as a whole kept this culture, where we all were available in the lab for each other. We were also welcome to stop by our advisor's office whenever something needed to be discussed, as he kept an open door. I found this to be very positive and enabled faster progress on our projects.