What is the difference between nightfall and masturbation? and after how much days masturbation is healthy ?

Nightfall vs. mastur: Nightfall is involuntary sperm emission at night during sleep, whereas masturbation is voluntary act of delivering self pleasure. I cannot comment on "healthy masturbation" because different people have different ideas of it. In my own opinion it is not healthy, so i would abstain.

I think you're fine: Almost all men enjoy their own bodies when they're alone. There's no physical health problems that result from it, and I believe it prepares you for adult life as a husband and father and keeps you comfortable so you can do other things. Why not think about that one special person with whom you may share a lifetime of love, rather than trash you find online? Maybe you can talk with Dad about this.