I was taught many years ago that you selected carb heat on as part of the downwind pre landing checks and left it on until within gliding range of the threshold. Only turn it off because its unfiltered air so as not to get dust into the engine.
In cruise select it as part of the regular scan check and if any of the signs as above. Leave it on for at least 30 seconds. If the engine runs rough you are burning water! Wait for it to smooth out before closing.
There should be plenty of heat in the exhaust system to melt the ice unless you are in a long slow descent when warming the engine every thousand feet is required for the engine's health as well as the carb heat!