I Had My Periods On The 29th Dec. Had Protected

I had my periods on the 29th dec. Had protected sex on the 14th Jan. As a precaution on15th morning (about 12 hrs) after sex took the ipill.
Usually I have a 28 day cycle, So I should have got my periods on the 25th Jan. Today is the 26th Jan, no signs of periods. I did not get any bleeding after taking the ipill.
1. How long are the periods usually late after taking an ipill?