I thought that a cat had to be at at least 700°C to work, wich mean being right red. I think that 20 minuts are too much, but for sure 3 minuts of calm city driving aren't enough.
I read somewhere that Ford had to program the ECU of the Focus RS sold in Canada to keep the idle at 1500rpm until the engine wasn't warm enough, or for a period of time to allow the cat to warm anyway. The magazine I was reading told that it took almost 10 min to the car to slow down the idle, when starting from cold