1. I used viagra for fun for some times, now I cannot get an erection without it. Anybody knows if it can harm the system permanently, and is this adictive
2. If I relay on it, how long will it be working. I mean it will be working for rest of life or after sometime it will get unaffective for me

it is not addictive.

its all in your head.

viagra doesnt do ANYTHING to create an erection, so CANNOT affect your ability to get one. Its a simple chemical inhibitor that reduces your body's ability to produce a certain chemical that breaks down cGMP, something your body produces naturally when you get excited.