Different cultures end the day at different points. The French, for example, tend to eat supper later than Americans do, and so their period designated as “evening” extends later than it does for Americans. Such differences can make it difficult to distinguish evening from night.
Most English speakers would not use “Good night” as a greeting, regardless of the hour. When you meet someone after dark and you intend to spend time talking with them, you should use, “Good evening” as your greeting, and “Good night” as your parting, if you are not likely to see them again that night. Radio hosts follow this pattern on their programs, saying, “Good evening, everyone” when they open the show and something like “Have a good night” when they close it.
In the home, family members do not greet each other formally with “Good afternoon” or “Good evening,” though they sometimes bid each other “Good morning” when they see each other for the first time that day. They also use “Good night” when they retire for bed.
“Good evening” is sometimes used in parting when it is recognized that only the interview, and not the night’s activities, has concluded. For instance, a restaurant server might bid his customers a good evening after collecting the check, because he knows that they are unlikely to go straight home and to bed after dinner. So, while 7:30 is probably “evening” for most, whether you bid someone a good evening or night depends on the circumstances.