Actually there is no guarantee of 8 days off in a month, I think the minimum they can give you is 7. I have even been down at 6 days off as they decided to count one of my days off down-route, as a legal day off, which they are allowed to do.
However the reality is a wee bit better than that most of the time. It also depends on what fleet you are on. If I was to average out my days off over the last 18 months or so (since we started doing 900 hours a year) I would guess I get about 10 days off pcm.
However, keep in mind terms and conditions only ever deteriorate here, they NEVER improve.