So if you enable the feature, it sends your photos to MS to scan... If you turn it off, they delete that data, meaning if you turn it on again, they have to process the photos again. Every time you enable it, you are using server resources.
However, this should mean that they don't let you re-enable it after you turn it off 3 times, not that you can't turn it off if you have enabled it 3 times.
all facial grouping data will be permanently removed within 30 days