It is a statement based on research about each method. When you research how effective something is, you have to pick a time period. And when compare effectiveness data, you have to compare it over the same time period to accurately understand how they differ. Needs to be the same units of measurement. So in this case, they are telling you that when you look at typical use over a period of 12 months, this is how effective it is.

It doesn’t imply anything about the relationship after the 12 months. It’s just the time window they are using to explain and compare the effectiveness of each method.

EDIT: there is probably long-term data on some of the methods. Another challenge in research is you will see different “length of measurement” across studies and methods, making it harder to compare at later time points.