Reserve is 12 days off, regardless of 30 or 31 day month.
The most consecutive reserve days on you can do is 6 in a row.
The best you can do is in a 30 day month, a chain of 6 on, 1 off, 6 on, 1 off, 6 on, and 10 off (the first or last 10 days of the month). I have done this strategy a couple times, but it does require staffing to be abundant, in that both the available reserves for the day total has to be greater than min required, AND, staffing in your RAP zone greater than min required. A 31 day month throws that strategy off because you can't do a 7 reserve day stretch. Then you could do something to get 9 days off in a row, and then stagger the remaining 3 days off into the month somewhere as a single day off.