Relax! Masturbation is a safe way to satisfy yourself. It would do no harm nor any illness. But there are many other activities where you can spend or utilize your time such as sports, games or pursue your hobbies. https://lovematters.in/hi/resource/men-masturbating
As for sex duration. try this technique and see if it helps you. Emphasize on your partner's body, and focus on your partner. Involve yourself in foreplay. If you are highly aroused, there is an increase in chances of premature ejaculation. So, you could also masturbate before sex, giving enough time for your body to get an erection again. Condom also helps in prevents premature ejaculation. Read the link given. https://lovematters.in/hi/news/premature-ejaculation-top-five-facts
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Relax! Masturbation is a safe