I don't have an exact answer, but I did just pay mine. My guess would be that it would be on or shortly after my cycle ends. Which for me is April 30... Has anyone noticed the timing who has paid lately?
It should update overnight online for a new digital certificate, with a new physical certificate going out in the mail (roughly 12 weeks from renewal to receiving). However, if your online dates are still wrong, please contact firstname.lastname@example.org with your ID # and they can fix this for you.