Purdue has not been ranked by the AP college football poll since September 30th, 2007. That week Purdue was 5-0 and coming off of a 33-19 win over Notre Dame. Purdue made it to No. 23 for a week before losing to Ohio State 23-7 in a night home game on October 6th.
It has not been back since. In fact, it hasn’t received a single vote, to my knowledge, since the October 7 poll. that has the Boilers approaching their record for longest time outside the AP poll. Purdue went from October 9, 1984 until October 20, 1997 without being ranked, a gap of a little over 13 years.