Gameinsky, you have probably overlooked that the comment was not made by the level author :) ... but if Mopsikus reads this, he should listen to your advice.
Lopardo, you have played no or very little P4M levels, so my first lesson is: play as much as possible, you have to see what is possible, what others already made. Then start experimenting with the level creator and you can ask anything you do not understand.
IMO 10 seconds to 30 seconds is an ideal length. If you want a level where people can fight for the highscore, 6-15 seconds is good. At any rate the best levels should look neat and/or have an interesting concept.
I don't think that's really that that you need. But P4M levels tend to go for a completion time of at least half a minute if done perfectly. Extend the level with similar challenges and different situations and you'll find that your levels will be more enjoyable :)
Lopardo, you have played no or very little P4M levels, so my first lesson is: play as much as possible, you have to see what is possible, what others already made. Then start experimenting with the level creator and you can ask anything you do not understand.