Cooling fans

Is there a general rule of thumb when the radiator cooling fans should come on! I'm not sure yet if I have a loose connection, defective relay etc. or just have to leave idling for more time! (Fans came on a couple of days ago but now seem reluctant to do so)
(Toyota Caldina ST195 1993.)

They generally come on when the coolant temperature is in the mid/ high 90's, and should go off again within about 10-15 seconds, its more of a concern if they take too long to go off, if the A/C is running they will operate differently depending on the setup, likewise some cars just run the fans at varying speeds instead of simply switching them on and off depending on the cooling needs at the time.Try perhaps running the engine at say 2-3000rpm (vehicle not moving) for a little while, that should get them to start up.Obviously if the engine is running too cold due to a stuck thermostat the fans won't operate either.

Try with air con on, this usually starts them up, otherwise do you have a overheating problem in the first place!Otherwise wouldn't worry about it.I've been there too waiting for the little suckers to come on.As franc123 said, they only come on for about 10-15 secs anyway.Cheers.

I have a Caldina as well although it's a diesel one, but not long ago did some engine work and replaced the radiator. Soon after it started overheating and blowing coolant out the bottle overflow. It seemed to me the fans weren't coming on when they should have. Turns out there was an airlock in the lower hose - the one that goes from the side of the water pump to the bottom of the radiator. After I sorted that, no more problems. The fans should come on after 5 minutes or so of idling (standing still) and run for about 15 - 30 seconds then shut off, and repeat this pattern. They don't come on at all when the car is moving as there is sufficient airflow through the radiator to keep the engine cool.
When the car is stationary, grab hold of the bottom radiator hose and check the temperature. It should be really hot - which means the thermostat has opened quite wide due to the heat. After having been driven for a bit, the temperature of this hose drops because the thermostat closes up because the radiator has dropped the overall temperature of the coolant.

Thanks guys. Yes I have had over heating problems, so am now paranoid!
New radiator, new water pump, new hoses, new top tank, new pressure cap etc.!.not all at the same time or in that order, but every time I fix one it puts pressure on somewhere else.
I will try a couple of your ideas tomorrow.