For me, since I have OBD and torque app,
I usually drive+warm up the car till the water temperature shows at least 50C,

For other cars, it's  usually the temperature bar has at least gone 2 out of 3 bars.
Or wait for the Blue light (for my Axia, which is after 50C) to go off.

Cause by turning on the aircond, obviously the radiator fan kicks in = engine warms up slower.
And added stress to cold engine.