How often do the divisional rivalry teams play for the AFC/NFC championship game?

Example: This year, the Chicago Bears are likely to play against the Green Bay Packers for the NFC Championship. Both of them are in the same division. I'd like to know how often does that happen. I don't want to hear any rematch in the wildcard or divional playoffall games. JUST AFC and NFC Championship games ONLY. Whoever has the best answers have to find an article and souces to be proved.