FFF
The 'alternate air' is really there to provide air in the case of a blocked filter (which may be due to impact ice for instance when flying in snow). Some aircraft have an automatic opening flap that just gets sucked open by the pressure drop, others have a pilot operated control.
Injected engines don't suffer from carb icing as its the cooling effect of the fuel through the carb venturi which allows the moisture in the air to freeze.
Continentals (as in the C150) have a much better carb heater system than Lycomings in a C152. The C150 has two silencers and a jacket around each. One provides hot air to the carb the other to the cabin for foot warming duties. The Lycoming set up is an open box which draws air over one exhaust down pipe.
The Continental system keeps a large amount of air in contact with a relatively large (compared to the Lycoming) area - the whole silencer.
Asa for how long you should leave carb heat on hot for - take an ice cube and play a hot air gun on it or poor boiling water over it - they're tough little things to destroy!
Read the AAIB reports and you will see how often aircraft 'land out' because of icing - that said, used often and for long enough, carb heat should prevent the onset of terminal ice.
As for waiting till the rpm drops. Remember that as the rpm drops, the exhaust cools and so does the temeperature of the air therefore reducing the effectiveness of the air to melt the ice - vicious circle time. When decending select 'hot' before reducing the rpm to ensure its clear of ice to start with.
Don't know why people insist on selecting 'cold' on final, if you have to go around push it in before opening the throttle - very easy on a C150, just stick your thumb out to collect the carb heat as you open the throttle!
I have yet to see an injector throttle body heater on a Lycoming or Continental - I stand to be corrected on that. Some Rotax engines have electric heaters on the carbs to get rid of carb icing.
Read the POH and abide by that is always a good place to start - there may be a specific comment regarding the use of partial heat - sometimes a bit of heat will bring the temperature to the right level to induce icing.
Whirly, isn't there some funny thing about R22s and use of carb heat?
Be aware, be very aware - or fly injected! Airflow Performance rocks.......