I'm hoping that your question crosses the radars of those who write about religious histories as I suspect they can speak to the role of this week's theme of Ritual Spaces as it relates to menstruation. That said, there is also a bunch of complicated, interdisciplinary, and fascinating secular history we can look at.

First to the issue of quantity: there are historians of sex, gender, and cultural norms, most notably Alice Dreger, who highlight that a significant difference between women in the modern era and pre-modern women is related to pregnancy, childbirth, and breastfeeding. That is, taken as a whole, people who can get pregnant used to get pregnant more often than they do now - which would decrease the number of times their uterus would need to shed its lining - as there were fewer options for limiting or reducing pregnancy. Cultural anthropologists, including Beverly Strassmann, have found similar patterns among women of the modern era who live in communities with higher birth rates and use few, if any, pharmacological interventions for spacing or limiting births. (There is a well-documented history of people using breastfeeding as a form of birth control.) Which is to say, that it's very likely that pre-modern women had fewer periods than modern women, but not because of biological changes over time, but because of sociological ones.

Second to the matter of "monthly": As far as I can tell, regardless of where or when we're talking in recorded human history, it seems reasonable to conclude the uteruses of people of childbearing age have always shed their linings over 3-7 days at the beginning of a cycle the length of a lunar or Gregorian calendar month (28-32 days.) The most compelling evidence for this is in how people write or talk about the topic. As you pointed out, the English word menstruation (as well as menses [(n.) "monthly discharge of blood from the uterus," 1590s, from Latin menses, plural of mensis "month"]) is related to the word for month. We also see this in idioms and phrases people use in casual or formal - non-medical - settings to describe the beginning of the cycle. They often incorporate two timeframes - month and week. (My personal favorite is the German term Erdbeerwoche meaning "strawberry week." My least favorite is the American phrase from the 1800s: "monthly illness.") However, just like the lunar and calendar months vary, the length of people's cycles vary based on a number of factors including genetics, diet, age, etc. Unfortunately, there's no way to know precise numbers as the concept of tracking such granular specifics is fairly new.

American educator and ethnomathematician Claudia Zaslavsky speculated in 1993 (and said at the time that it was more of a musing than a conclusion) that an artifact known as "the Ishango bone," recognized as the oldest known evidence of intentional tallying by a human, may be an early version of a period tracker. It's worth stating that since she made that claim, our understanding of how early humans lived in communities and how labor was divided along gender lines has shifted and there's no reason to think a woman made the notches to document days between specific phases of her cycle. Another AH user, though, may be able to chime in on ways people throughout history have documented the menstrual cycle and what we can learn from their efforts.

My hunch, though, is that history is going to be hard to find and that's mostly because of the topic at hand and whose history we're looking at. In her answer to a question on pregnant women and corsets, /u/mimicofmodes describes how social norms were such that even talking about being pregnant was considered impolite for women of certain social classes. (I get into something similar with regards to the history of bathrooms here.) This isn't to say there's no evidence, but rather historians have to have a deep understanding of the historical record to interpret the conversations around such topics.

One useful example of this is the related topic of abortion. A few years ago, an NPR reporter claimed she hadn't found any examples of advertisements for abortion in newspapers. Historians of women's health, including one of my favorite historians, Lauren MacIvor Thompson, immediately raised a red card and pointed to dozens of examples and how the modern reader has to understand the context of such ads. (NPR issued a correction. I get a bit more into this history here.) And, to circle back to your question, part of their explanations included pointing to ads that advertised ways of balancing a woman's monthly flow or helping her restore a flow that had stopped. In her fantastic book, The Myth of the Perfect Pregnancy: A History of Miscarriage in America, Lara Freidenfelds offers a similar context. She explains how white women didn't consider themselves "pregnant" just because their uterus didn't shed its lining (they, of course, didn't use such language) for a few months. Rather, the lack of blood was commonly seen as an interruption or pause in their cycle that might be started again by nature - which helps us understand that our current approach to miscarriage in America is very much shaped by modern sentiments - or by an intervention - which is to say, an abortion. If and when it didn't start again and she noticed other changes, including the feeling of the fetus moving about, only then would she become an expectant mother, which, depending on her social class, may or may not have changed how she moved through the world.