Hopefully someone knows this without having to look it up (I can't find it at the moment).
It's quite hot: 100 degrees when the car isn't moving, 95 degrees if you're doing less than 50mph, and 105 degrees when you're doing more than 50mph; or something like that. - Hot enough that I've only seen my fans turning a handful of times; I thought they were broken until I opened the bonnet and saw them spinning-down on a hot day.
I think Ed has pointed-out that if you've got a speedo delimiter fitted that changes the speed the ECU sees (so the 180kmh limit becomes 180mph), that will cause the ECU to not adjust when the fans come on... so you'd need to do 80mph before it changes. - I'm not sure if that still applies to converters that don't delimit the car (which is what I've got) because the ECU can clearly see the real road speed.