Oh Amy,
This is the toughest question on the board. Nobody has reported more than two regular natural cycles on PM so far. Men take PM continuously. So did MydreamisGcup on the depo provera shot, and idreamofD still does on the Nuvaring. Run.Rabbit's program thread is a nice example of a cycle that works well with a bcp.
Women with a natural cycle on PM can keep it going for two months or so. Then their cycles become like the cycles described in the publication I linked at the bottom of this post:
http://www.breastnexus.com/showthread.php?tid=8419&pid=41574#pid41574
They are either very long, 36-42 days, or very short, 17-21 days. One woman can have both long and short ones, and sometimes what happens after day 17 is more like spotting. According to that publication, the cause is a second ovulation, which occurs at the end of the period.
Because PM suppresses ovulation, it's important that you either start PM immediately after the end of your period, or you are already taking it. For this reason, I have always recommended to start PM at 1 capsule on day one of the period. After the period, increase to 2, and if you can take it without headaches, increase to 3 on day 9.
Stopping or decreasing PM triggers ovulation. Therefore, I think it's important to stop, or go down to 1 capsule, on day 12. Ovulation normally follows on day 14. Some women feel it, some women use the thermometer. After ovulation, it's OK to take two capsules up to and including day 24, as long as you ensure that you are not becoming estrogen dominant. Some women do this by taking a phyto-progestin, like fennel tea or turmeric, others take fenugreek or wild yam to increase natural progesterone.
What you describe, starting PM after the period for 15 days, is Bella's schedule, named after a vendor of PM. BelaBrasil and aimingford did this successfully for two months, but then their cycles became irregular too.
From day 25 to the start of the period, you have to go by the symptoms. If you get a headache, you are estrogen dominant, and if you have bad PMS, you lack progesterone. In both cases, continue a low dose of fenugreek, or wild yam, or fennel or turmeric. Chaste tree berry (vitex agnus castus) may work too. Don't start PM again before your period, unless you feel really progesterone dominant (clear thinking, easily irritated).
In a long cycle, the days from day 25 to the start of the period are not funny. You feel like you are pregnant, have PMS, are worried, want a pregnancy test, and are estrogen dominant (headache, depressed, foggy thinking, low libido), all at the same time. Most women do a cycle without PM after that experience.