If a woman is using birth control pills (after consulting with a rabbi) with a longer-than-one-month cycle, when are her onot perisha? For example, suppose the woman becomes nidda every 3 or 4 months; does she still observe the onot perisha for the day of the month, or the 30 days? What other things must be considered in such a situation?
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If a woman has no set time of the month but her period always occurs more than x amount of days after her previous period, then for that amount of days she is considered to be awaiting a veset kavu'a - a regular cycle, and she need not observe any days of separation during that time. This is known as the veset of the Terumas Hadeshen, and is mentioned in Shulchan Aruch YD 186:3.* (That one who has a regular cycle need not observe any days of separation in between is found in 184:1.)
The definition of 'always' is the just like every veset; three consecutive times Halachically defines a regular cycle.
*See Pischei Teshuva 187:26 who mentions this particular application of the Terumas Hadeshen's veset, and cites the Chavos Da'as who also says so. See also the Sidrei Tahara 184:4 who mentions it in passing.
In Torat HaTahara 2:14, R' David Yosef rules "A woman who takes pills to push off her period [i.e. so that it's longer than veset ha'haflagah or veset hachodesh she established from previous periods], min hadin her husband doesn't need to separate from her at the the time of her (previously) expected veset, because in our days it's proven beyond all doubt that these pills actually do push off her period. But someone who is machmir should be blessed."
R' Mordechai Eliyahu addresses this question in detail at the end of chapter 7 of his sefer Darchei Tahorah. I'm not going to quote him in detail because it's long. In short: she doesn't need to be concerned about her old period, and she needs to be separate starting 3 days after she stops taking the pills until 7 days after she stops taking the pills (temporarily), though she can establish a veset kavua based on the number of days after she stops taking the pills, and then she only needs to separate then. He discusses in detail what happens when she stops the regimen of pills completely, and other situations related to pills.
According to R' D. Feinstein (I heard personally), predictions of what the pill will do are irrelevant. The law goes by when menstruation happens or doesn't happen. [This psak was not regarding multiple months, but a "28 day" pill. There is no 28th day onah because "How do you know?"]
So until there is a basis (chazaka) otherwise, she must keep that date of the next month.
Obviously, once the 30-day general interval passes or her personal interval from the last incidence, you don't need to worry about those anymore (there is no 60-day interval).