How long does it take to play Diplomacy?
Odds are that you're not going to want to get it (which is tragic, as it's a great game), since it's a long-play game.  I've found that a game typically takes 4 hours or more.

Interestingly, new player groups and experienced player groups will typically take about the same amount of time.  New players take time because they're figuring out the rules, but they're not going to be nearly as Machiavellian.  Experienced players take time because they're creating plots within plots and being devious, which leads to protracted engagements on multiple fronts.

If you really want to make a game of Diplomacy cruise along, I recommend the following:

1) Have an experienced player resolve all orders at the end of each turn.  That helps speed things along.

2) Set a solid time limit for each turn, like five minutes.  Five minutes is very long for the first three or four turns, but by the eighth turn it's barely enough time to write your orders.  Yes, setting a time limit impairs your ability to engage in negotiations, but it'll make the game end in a reasonable amount of time.

Another option:  Play in one-hour (or two-hour) sessions.  Leave the board set up between sessions.  Long ago, I made a copy of the board, mounted it on foam-core, and stuck color-coded push-pins in it to "save" the state of the board between games.  Something nice about this is that you can do all the diplomacy and negotiation you want to, and you can engage in diplomacy between games.  That's a big part of Diplomacy (go figure), and can make for a really fun experience.