There is no cut off, but I would just allow common sense lead you...you shouldn't say good afternoon at 8pm or good evening at 2pm....I think I would stop using good afternoon around 4pm and start saying goo evening around 6pm in between I wouldn't say either. Maybe good day. Keep in mind these aren't rules just my own personal use of these phrases

I'd go by whether the sun has set or not.

There is no actual cut off time.