Unfortunately they don't give any details on what constitutes more frequent and less frequent. For example once a month vs once a year or once very 2 weeks vs once every 2 years, etc.

Whole blood donation generally is only allowed every 8 weeks. Double-red blood cell donation is allowed every 16 weeks up to a maximum amount per 12-month period. I believe Plasma can be safely donated every 4 weeks. Doesn't tell you specifics about the study, but know there are different kinds of blood donation and different frequencies that are permitted for each.

I think it's common for regular donors to donate almost as often as is recommended.