Many are fearful of the time commitment that blogging involves. While it is certain that the posts won’t write themselves, YOU set your schedule. If you feel you can only blog once per week, that’s ok. If you feel you can blog once per month, that’s ok, too.
I would suggest that weekly might be more effective but monthly is at least a start. Once you become more comfortable, you may decide to increase your frequency. Starting slow is fine, but stay as consistent as possible with your timing, so that your readers know what to expect.