dude i am 16 now and i had the same problem where i would masturbate for a while and was unable to cum, i once even masturbated for 20 minutes with no luck. i had watched porn before but i had never mastubated to it, then about a year ago while my parents where gone all day i was watching some, and sicne no one was home i decided to go at it while watching something, and that was the first time i ever came but i have had no trouble since even when i dont watch anything. so you may want to watch porn while you do it.
it will most likely take between 5 and 10 minutes.
Dude our normal if u start masturbating often you will find it takes time..... The longest I've ever taken to cum was over and hour. I think that the first time i have sex it will be a different story..... but then again the first time i ever tried it i was able to cum in like 10 min. every one is different so Ur fine