The default answer
Once a month is a billing cycle, not a finding.
Ask the internet how often to book and one number comes back: monthly. It's a tidy answer, and it usually has the shape of a recurring charge — every four weeks, a slot somebody would like kept full. That doesn't make it wrong. It does mean the number came out of accounting, not a trial.
The research is messier. NCCIH's plain-language overview of massage therapy is careful about what the evidence will carry: studies that are often small or low in quality, and benefits reported over weeks, not years. None of that gives you an interval.
So start one step earlier, with the question people skip. Why are you booking? A shoulder that's hurt since March and a Friday you'd like to survive are different problems, and they don't want the same calendar. Get that part right and the interval mostly falls out of it.