Once or twice a week is fine. Take care

Limit your masturbation as low as possible. Yes it will harm your health in a long run.

Masturbation is a natural process when a person is getting sexually active and 80% of the boys start it to enjoy before the age of 14 and rest try it after age of 14. It is not harmful if it is done once or twice a month but excess of everything is bad. Watching porn movies and ending up in Masturbation is  harmful phenomenon as it weakens the Muscular Strength of Penis and induces Early Discharge at slight arousal. You can control this mindset by doing Pranayaam, Dhyan and Meditation when you feel an intense urge to masturbate. This is very helpful and it can entirely change your bad habit. Penile hardness is very important factor in the Treatment of Erectile Dysfunction. It can be achieved by taking Nerve  tonics like Ashwagandha Powder 1Tsf twice daily with Dhatupaushtik Churan I Tsf at night with Warm milk. Slow massage on the shaft of Penis with Shri Gopal Oil would also help. Detailed medical history is required for evaluation before giving you a good prescription.