The more players Monopoly has the longer it will go on because the whole point is to run your enemies out of business. Ofcourse at the same time, the fate is in the dice. In theory you could finish a game in 2 hours provided you bought the best places and your opponents just always happened to land on your properties.
People always say that about Monopoly, but as long as people are willing to trade with each other to get monopolies it shouldn't really take more than 2-3 hours...
This. Trading and deal-making with your opponents is not only permitted by the official game rules, it's all but an essential part of Monopoly. Extremely rare indeed is the game where you can win without cutting some kind of deal with another player for his or her properties.
People also tend to add a lot of house rules that, while fun, only increase playtime in the long run. The worst offender in this category is the "Free Parking Jackpot" rule (where all fines and taxes are paid into a pot and the first person to land on Free Parking can claim it all). It's a nice rule, one I play with myself, but it does allow for more money to be in the game -- and since the game is supposed to be set up so that every player in the game bar one ends up bankrupt, it actually ends up making it take longer for a player to go bankrupt, and thus the game itself takes longer to play out. On the other hand, it can make the game more exciting by adding that extra wildcard factor which can save someone's game (maybe even your own), so it's certainly worth thinking over.
