The only problem with extending the number of days you work is the fatigue issue.
7 12 hour night shifts in a row is too much for anyone - long term, you will do yourself harm, even if it is great for time off.
The smimple 4 on, 4 off, 12 hours (4 days, 4 off, 4 nights, 4 off) works best in the end. It doesnt take too much out of your body, it is easily predictable etc.
Variations on it are 2 days, 2 nights, 4 off, but that is crap as it means you always finish on a night and always go back on an early, so your first day off, isnt really a day off as you work through that night into the morning.
Another pattern tried is a twist on the 4 on 4 off, which involves 5 on, 5 off, and then a 3 on, 3 off, which if worked correctly, guarantees a certain amount of saturday/sunday periods during each month. But, it can be a bit unpredicatable in terms of forward visibility.
My advice - dont change what works well!
Shift work is very hard on the body (as you will learn after a few years doing it - for one, it is a major cause of depression) so you do have to take that into account, as well as the time off benefits etc.
Working 84 hours straight, on nights, is too much for anybody to do regularly!