How to observe niddah and vesatot while using hormonal contraception is a matter of halachic debate. Our opinion is that, if a woman does not become niddah during the first three months on Seasonale, then her next halachic obligation is to treat the days when she goes off the active pills as onot veset. Now that you are already in niddah, you will wait at least five days, count seven clean days, and immerse. So long as you do not bleed, you will have no further niddah-related requirements until the end of the next package cycle, when you should abstain until becoming niddah.
After you have used Seasonale for three multi-month cycles, you will not be required to abstain from the time you cease active pills. Instead, you will begin to observe onot veset from the earliest day or night when you might become niddah (based on experience) after cessation of active pills.
If you experience a consistent interval between cessation of active pills and onset of bleeding three consecutive times, then you will have established a veset kavua.