Hey Sri. I don't think there is an official time for dividing evening and night but usually "evening" is the time when it starts getting dark outside and before you go to bed. For instance 6-10pm would be a good estimate, anytime after that and while you are asleep would be considered ''night''.
Just remember ''evening'' comes before ''night''. "Evening'' also sounds a bit more formal in certain contexts.
Also, when used as a greeting, ''good evening'' is usually used to say ''hi'' and ''good night'' is more of a farewell.
Hope that helped. :)