Every 15 minutes and at the end of your conversation.
As far as the other etiquette issues, pretty much what the previous replies have noted. As far as calling other users, Callsign or name or nickname, whatever gets their attention if they're listening. Some areas have active simplex and repeaters in use, but for the most part GMRS is a "bring your own" type of service. In my case it's my wife and son using it when up north away from cell phone service, around the large city lot at home and with a small number of friends near home we chat with occasionally. I rarely use repeaters.
Have fun however you use it, and don't overthink it or take too much umbrage if some self appointed radio cop gives you grief if you do "something wrong."