Is it mentaly normal for a person to watch pornography regularly ?

Not ideal: Pornography is compelling because watching it induces Dopamine release in the part of our brain that experiences reward. It is an easy problem to fall into, and the proliferation of adult sites is a testimony to its popularity. But it is a complete waste of time. It fosters dissatisfaction through increasing awareness of what you don't have. Focus instead on what you do have, and enjoy it.

Talk with someone: All men enjoy seeing the human body displayed tastefully, in its natural beauty ("erotica"). Most of us do not think this does us, or society, or our relationships any harm, though some "theorists" disagree. If you find yourself obsessed with images that are degrading, or your thoughts bother you, or you are unable to function as an adult because of some compulsion, then get with your physician.

Up for Debate: The issue with your question is: what is “regularly?” Ten times a day? Probably not healthy. In order to look at this as a health issue and not a moral one, we usually ask things like: Is this behavior negatively impacting your life, do you engage in the behavior at inappropriate times (i.e. at work) or spend significant money on it, and do YOU feel like it is a problem?