Monthly is the default, and defaults exist because they mostly work. But the right cadence depends on the length of the book and the free time of the person with the least of it.
Monthly is the safe answer
Four weeks is long enough to finish a 300-page novel at sixty pages a week with a fortnight of slack for the week life goes wrong. That slack is what makes it survivable.
Fortnightly works for short books
If your group reads novellas and non-fiction under 200 pages, a fortnight keeps momentum without pressure. It also means a missed meeting costs you two weeks, not two months.
Weekly only works with chapters
Weekly meetings need a chapter-a-week structure and a group that genuinely enjoys the discipline. Most do not, and finding that out in month three is expensive.
The real test
Ask the busiest person in the group what they can manage, and use that number. A club moves at the speed of its slowest reader or it loses them.