It all depends on what you're planning on doing with the game.
1. Give it away for free? 10 is plenty.
2. Sell it for less then $5? 10 is good, but each level needs to take the player about 10 minutes to finish, otherwise the player will feel ripped off.
2. Sell it for more $5-$10? 10 is enough levels if you make sure each level takes over 10 minutes to finish. If not, add 2-5 levels to the game.
3. Sell it for over $10? 10 isn't enough unless each level takes far over 30 minutes to finish. Since that is doubtfull with a FPSC level, you have to shoot for at least 20-25 levels.
With each game, I'm shooting for at least 10 levels and at least 2 hours of game play. Sometimes I do end up with a less levels, but then it takes a long time to finish each level. Often I add more levels to it. My goal is most of the time the $5-$10 market segment.
Just going back to the length once more. You can easily obtain a long playing time for the level, by building a complax maze like level and let the player walk around for a long time. BUT that isn't the idea of a long playing time. That is boring and the player looses interest pretty soon. The player needs to encounter action of some sort, be it through enemies or smaller objectives he has to accomplish. People rather play a 10 minute action packed level, then 30 minutes with hardly any action at all.
With some of the FPSC games I've tried, boring has come straight away to mind. Nothing happening at all. Walking around a level, searching for a key for over 5 minutes, with nothing else happening and the same pattern repeated time after time. THAT IS BORING. That will kill the succes of your game.
Make the levels entertaining, things happening and then even a shorter level is well worth playing.
