What is the average number of turns in a game of Monopoly?
Playing online against 3 computer opponents, one of my games would typically last 30-45 turns. I used the standard rules (i.e., no money on Free Parking, limited number of houses and hotels). Because it was me against computers, the games would take ~15 minutes to play. 

Playing against human opponents would certainly extend the actual playing time, and may likely increase the number of turns for a game as well. House rules which inject money into the players' hands (e.g., money on Free Parking) will increase the number of turns, and thus playing time, too.