an hour? sheesh, I don't know about you, but my 8 level game at this point to the average FPS player will take an hour. with 50 levels, done right, every drop of gameplay squeezed out of them, an FPS game you make could be easily 40 hours or more to someone who hasn't played it. the trick is: getting people to stay in one place longer.
as kravenwolf said, backtracking is a great idea, especially if enemies spawn behind you, not only that, but there could be more things to find. let me give you an example.
1) a level taking up 1/4 of the grid, start to finish, no keyes or puzzles, just enemies- 8 minutes to finish
2) a level taking up 1/4 ot the grid, a linear path start to finish, several keyes and puzzles (be they jumping or otherwise) along with enemies- 16 minutes to finish, depending on how many hints are given to the player.
3) a level taking up 1/4 of the grid, likely a few different layers tall, a start room, a finish room, enemies, doors to unlock where you find keys to use switches that open garages halfway across the map to the next section of the level, plus jumping puzzles, plus enemies, plus some explosions and action to keep the player intersted, and a win room- 25 minutes to finish. depending upon hints given to the player.
1) pros:
keeps player interested
constant action
fast paced, feeds game to player in small tidbits.
1) cons:
way too fast
entire game will last 4 and a half hours if fifty levels are used and each level is stretched and enemy infested.
killing can get repetative.
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2) pros:
perfect length to get lots of gameplay if used in a linear game
a good balance of action and navigation
feeds game to the player in digestible sized morsels. nothing too daunting.
MAXIMUM squeezage is a good 9 hours of gameplay
2) cons:
can still be seen as linear
puzzles AND enemies make player have to try again, and dying isn't really... well... fun to anyone.
can still be a little repetative if the same formula for levels is used (IE start>find key>fight>backtrack>find key>open new door>fight>end level)
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3) pros:
will never be seen as linear
can have more than 20 hours of gameplay if done right
large open levels are more fun to explore than small cramped levels
player will feel as if they have more freedom.
3) cons:
game will seem to have massive, daunting tasks placed on the player
system requirements soar through the roof with big levels that have enemies
still can get repetative
long loading times
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finding a perfect mixture between all of these can lead to some great games with lots of gameplay. of course, I don't know how much time you have on your hands, and all gameplay hour stats were calculated from base number averages given to me by my friends in different test games I gave them.
obviously, gameplay is defined by you. if you have the time on your hands to design 50 levels that take up every inch of grid space, then more power to you. that would be an 83 hour 33 minute game. then again, it is all based upon skill of the player, and many other factors. I'm just here to say no- FPS creator is capable of making games that have more gameplay than the average commercial FPS. 14 times longer when averaged the amount of time I took to play MW2, MW1, TCR6V2, TCR6V1, SoF

B.
end result- when it comes to gameplay, and amount of hours possible to squeeze out of a game, FPSC is a pretty formidable engine in that regard.
add me on skype- isaacpreston. WWC percentage complete: 60%