Normally, after adequate stimulation one reachs climax. The climax consist of orgasm(intense feeling of pleasure) and ejaculation(discharge of semen). Usually it takes around 15 min to 30 min to reach the climax. If you are having such an issue, make sure you are masturbating in a correct way.
Next Steps
You may consult with a sexologist. You put up a detailed query here on practo or visit one nearby you.
Health Tips
Shouldn't be a major problem.