The Roe fan kit works well. But just leaving it on, while the car is not running, will take it's toll on the battery, and take a long time to cool down the engine.
I do not have a timer switch on mine, but may install one later.
What I do is let the fan run for a couple minutes after the car is shut down, and then go back out, restart the car, cycle the cool water through the engine, and then the temp will really shoot down.
Just letting it run, without a restart, will take a LONG time to cool down the engine. And may kill your battery too! Although, the water in the radiator will be nice and COOL!