Well my car overheats at idle but the fans does come on. I timed them and at normal temp. They come on for about 30 seconds and about every 2 minutes. However the fans never come on when I turn on the ac nor do they come on when the car is turned off after overheating. I have tried using a civic radiator I had with a new cap and still the same thing. I changed the t-stat and the radiator fan switch on the thermo. My coolant level is full and the system has no air in it. I am thinking bad waterpump?

 

The fans don't really do much. That's not the problem.

How long does it take for it to start overheating?

A few quick things to check/change:
Thermostat
Bleed the coolant

The thermostat would be the first thing I would change.

 

thermostat or a air in the coolant lines