If I'm calling from a payphone, I will not let it ring more than 3 times, because usually answering machines pick up on the 4th ring and the phone swallows my coins. I would rather call them three times, 3 rings apiece.
If I am using a regular phone, I let it ring AT LEAST 10 times before hanging up.
When I receive a call, I let it ring at least 2 times so that the call would register on the caller ID.