Ideally instead of ejaculating then waiting to get hard again, you are able to control yourself so you only ejaculate ONCE. You want to keep your arousal level between 65% - 75% so you don't overstimulate your brain with sexual thoughts and feelings, which makes you ejaculate early. Do this by focusing on your partner in a non-sexual way. Focus on understanding her as a woman and building the emotional connection between you.
It took around 20 minutes to get erection back, some times 30 minutes. After erection I ejaculate within a minute or 2. Is there any effective way to retain erection for long and stimulate after getting complete satisfaction?
That all depends on your focus on a woman during sex. Masturbation in itself does not cause Premature Ejaculation. How long do you last when you masturbate?
No, this does not mean you will have PE in the future.