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.
boardgames.stackexchange.com/users/612/invisiblejon
I would say that its about 5 hours on average.
Its not that the game ends at that point so much as the game is agreed to be a foregone conclusion by that point, even when it might not have been, due to fatigue/disillusionment from the players :P.
boardgames.stackexchange.com/users/71/GWLlosa
As mentioned in other answers, 4-5 hours is an accurate average length.
I'd also go as far to say that 1/2 of all games are never 'officially' complete - more often a leading player and assumed winner is agreed upon.
I'd argue strongly that neither points detract from the game
, it's length and involvement is half the fun - but if you don't feel you have that sort of time to invest, it may be worth avoiding.
It's worth pointing out that Diplomacy has a lot less pieces to wrestle with than Axis & Allies and to a certain extent, an easier ruleset to learn.
Its slow point is diplomacy, which is easier to tweak, rather than the (very fiddly) physical move phases of Axis & Allies.
boardgames.stackexchange.com/users/73/Jon Hadley
As the consensus opinion is: yes, it definitely will take you a whole day to get through a game of Diplomacy - why not play it as a PBM (or PBEM, or whatever kids are calling it these days) instead?
Just because something comes with a physical board doesn't mean you have to gather your friends and sit round a table.
A Diplomacy game is quite an undertaking, but you needn't complete it in one gruelling sitting.
Amortize the endless wrangling and backroom deals over a period of weeks - the thrill of finally backstabbing your "dearest allies" will stay just as sweet!
boardgames.stackexchange.com/users/436/thesunneversets
My experiences have been varied, but much like @GWLlosa, I have found it to be, at a minimum, 5 hours.
We often take a weekend day - say a Saturday or a Friday holiday off of work - to play the game.
This way we can start around 10 AM, have lunch in the middle, dinner later and then stop whenever people are done.
It is a fun, quality game, just one I never plan on finishing.
That being said, I have heard of (but never been a part of) cut throat groups where the game goes much faster, but I think you have to have a group of people who are willing to backstab at the right times, and are willing to betray alliances for a win, something that tends to be less common amongst friends in my experience.
boardgames.stackexchange.com/users/422/aperkins
I guess this varies a lot from game to game, but I've seen everything from 120 to 360 minutes, and wondered where the average lies.  If it's closer to 360, I don't think I will bother to buy it. I know how rarely Axis & Allies is taken out of its box...
How long does it take to play Diplomacy?
boardgames.stackexchange.com
<diplomacy><play-time>
boardgames.stackexchange.com/questions/1967
boardgames.stackexchange.com/users/469/erikric
An average length is 4-5 hours. Try taking a weekend to play the game. 4-5 hours is an accurate average length of the game. Many take a whole day to get through a game of Diplomacy and dedicate a weekend day to play the game with breaks in between.
4-5 hours is an accurate average length of the game. Many take a whole day to get through a game of Diplomacy and dedicate a weekend day to play the game with breaks in between.