When I was 15 I had the problem of ejaculating very quickly. For those of you that want to increase your time, here is an easy method that I used that works great.
masturbate until you have an erection, then stop for about 15-30 sec. then masturbate until you are about to ejaculate, try to find the point at which you can stop before you actually do. Masterbate to the ejaculation point, then stop for about 15-30 sec. then repeat the process, you wont be able to do it many times to start with, but after awhile it will start to work. Try to get to about 6-8 cycles through.
I used to ejaculate in about 3-5 min, after doing this for about 4 weeks I was able to go 20-30 min.
One of the things you have to remember when masturbating is that it wont be exactly that long when you actually have sex. When you masturbate you know what will stimulate you more, and also you are rubbing the foreskin faster then you would in sex. So the time that you will have with a women will be different then how long you masturbate, it will generally be 2-4 times longer.
The average man takes about 5 min. to masturbate and about 15 min. during sex, so if you are taking 20-40 min. like some of you are saying. Then you may have a problem, or you are trying to sound good.
Note: This method is only one of many that you can try, it is not for everyone and may not work for certain people.