Never
There are no rules stating that a woman could not play in the NFL, but there has never been a woman to play in the NFL. There have been several females that have played the placekicker position in college football.
NFL player Robert McClain played for Connecticut.
NFL player Michael Floyd played for Notre Dame.
NFL player Charles Woodson played for Michigan.
NFL player Joe Thomas played for Wisconsin.
NFL player Zac Robinson played for Oklahoma State.
NFL player Kyle Wilber played for Wake Forest.
NFL player Rob Sims played for Ohio State.
NFL player Kyle Juszczyk played for Harvard.
NFL player Jimmy Smith played for Colorado.
NFL player Travis Kelce played for Cincinnati.
NFL player Jacques Smith played for Tennessee.