Bonjour or bonsoir ???

When does one say bonjour or bonsoir? We know bonjour means good morning and bonsoir means good evening but in the absence of good afternoon... when does bonsoir take over from bonjour?

It's been over two years since I have been living in France and I cannot really tell when bonsoir is appropriate.

Learning French in Nigeria, I was told that :

• Good morning = bonjour
• Good afternoon = bon après midi
• Good evening = bonsoir

Well since I have been here, people say bonjour to me at night, and I have never heard of bon après midi as a greeting. When I take lifts with my neighbours in the afternoons, whether it me leaving the lift first or it's them they often say to me bon appetit :) And I'm often like... who told them I am going to eat :D

Some French people told me bonsoir is used after 5pm while others said that I can use bonjour in the evening as far as I am seeing the person for the first time that day. But every time I say bonsoir to someone‎ I am replied with a bonjour and when I meet another person and say bonjour just because the last person replied me bonjour, he will reply me with a bonsoir.

It can be so frustrating... sometimes I feel they just want to say the opposite of what I say or is it that the French people themselves don't know which to use ?