Moonburn,
You might try the engine's manufacturer (Lycoming) manual. If I'm not mistaken, it's a tiny pink and white binder (size of a pocket book). There you have some general guidelines regarding the use of carburetor heat. Right now I don't remember if there's something on PA28 POH about use of carb heat.
Operationally speaking, when I was working as a CFI, the school SOP during power-off descents and approach was the following:
Power-off descents (including emergency simulation)
*Select carb heat on if OAT in the current altitude is below 20C;
*Idle power;
*At every 300ft (you may choose the interval), revs up to +- 2000 RPM and throttle to idle
Power-off approaches
*Select carb heat on if OAT < 20C when you're abeam touchdown point and;
*Retard the throttle to idle;
*On base leg (altitude loss of +- 300/400ft since beginning), revs up to +- 2000 rpm and then back again to idle;
*Land
*In case of go-around - full power > ensure proper flight path (the most important item, of course
) > check aircraft configuration > carb heat off
Power-on approaches
*Abeam touchdown point, RPM around 1500-1700 - no need to use carb heat
*Short final - throttle to idle (no carb heat, because it's a really short period of time)
OAT range there was from 0C up to 35-40C. Typical PA28 cruise levels with temperatures around -5C to 10C.
Hope this helps a little
