Conversation so far:
Did Bert Bell teach at Penn?
Bert Bell joined John Heisman's staff at Penn as an assistant coach in 1920.
What did Bert Bell coach or teach?
Football
How long did Bert Bell teach?
Several years
Did Bert Bell have any scandals while teaching?
Bert Bell was well regarded as a football coach.
What did Bert Bell do after teaching?
In 1928, Bert Bell tendered his resignation at Penn. Bert Bell was then an employee of the Ritz-Carlton in Philadelphia.

Question: How long was he enlisted?
Self-contained rewrite: How long was Bert Bell enlisted?