At Maastricht our roster had us working 4 on, 2 off and we were not allowed to work more than 8 consecutive days so we were also effectively limited to working 5 per month.
There was also a limit on the total allowed IIRC after one controller manged to disappear for a couple of months to work in the family restaurant in Nice and another (a PPRuNeR) phoned from the Caribbean that he was nable to travel due to having diahorrea. Now everything has to go through a roster office.