How do you determine the play length of your game?
Do you do this during the design process, or do you just map out the levels and the design you want, allowing the game to be however long it will be?
Is there an equation for factoring difficulty into the mix?
This is for single player games of course.
No one has replied yet? Oh… well I’ll chime in.
For us (and our big title hasn’t been announced yet) it was more about quality and replayability. Making a game with 20 hours of content isn’t feasible, and on a mobile device, its unrealistic to expect a user to hang around for that long anyway.
I personally use the 7 minute rule… If you can engage a user fully for seven minutes, you’ve done a good job. If you can make them come back time and time again, that’s all you need.
A $15 movie ticket works out to 8 minutes per $1 (2 hour movie)
A regular xbox game, which might give you 10 or 12 hours of engagement, works out to $1 per 6 minutes… (Australia currency works out to $100 for a full price game, adjust for your currency)
So really, for a $1 game, anything over 7 minutes is brilliant value for money… In my opinion 
(This all assumes that your game experience is good… Obviously, if you paid $1 for 7 minutes of poorly-made crap, people’s experience won’t be so good) 
Here is a good trick to try. ( assuming the game is a 1st/3rd person shooter or something similar control wise)
-Design one level of your game
-Time how long it takes to walk from start to finish, with no enemies or obstacles
-Then time again on the same level with enemies/obstacles/puzzles added
If it takes you twice as long to get through the level with enemies added, than for an 8 hour game, you need levels that take 4 hours to walk through unhindered.
If you know your characters walk speed, than you can figure out how long in meters( or w/e unit you are using) the games main path needs to be.
P.S. Its actually better to have someone else( friend/family) go through the level and time them. Otherwise you wont be able to take into account the time is takes to figure out which way to go. Because you, as the designer, already know which way to go.
P.S.S This also doesn’t take into account difficulty curve. As the player goes through the game they will be able to get through obstacles/enemies quicker. So as a designer your goal is to give the player more challenge to match their improving skills as the game progresses.