masturbating isn't harmful, It has no effect on sperm quality or quantity and it doesn't cause premature ejaculation. Just like masturbating, intercourse can last. If you ejaculate this much fast, that can cause a trouble when you do sex with a partner. Because women wont get enough pleasure before your ejaculation.

It is particularly important because as you know after ejaculation male's brain is in a retarding mood and cant even think about sex that time. You may need to rest for a few minutes and get into the business again with your partner's help, if she still wants sex. And 3 or  4 minutes sessions is not a good go at all. Because  intercourse duration can be as long as 2 or 3 hrs if both of you wish.

The best thing you can do is to train your mind to prolong the masturbation as much as possible with out ejaculation. I did have this situation of ejaculating only by 10 or 15 minutes' stroke but now i trained my mind to go for at least 2 or 3 hrs when I got a couple of hrs to spare.

For sure during this time period, sometimes you may reach the point of no return but you usually pause for a few moment thinking about things other than sex and continue again.

Remember sex is happening in our brain first, without it, we cant enjoy this priceless gift this much. So train your brain, or your mind exactly. Good luck friends  for as long as you want it to, once you become good at it.