Yeah as mentioned, prolonged cold start idling is no good due to the fuel washing down the cylinders. Especially when cold, since the fuel doesnt mix all that well (hence ECU runs rich for a lil bit). Also the extra fuel can end up in the engine oil, which is definitely not good. I've seen plenty of oil analysis where excessive fuel in the oil was due to overboard warmups (ie >5 minutes).
Futhermore, warming up the engine by gentle driving brings it up to operating temps alot faster than idling away. This is good in respect to distilling off condensation in the oil, in very cold climates.
I reckon 2 minutes maximum, but 1 minute should be plenty.