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

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.
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.