Do it once a year. Also I have been told my people that since we run soap thru our coils that also helps descale them. But once a year we run descaler thru them.
Best thing I have found to clean coils is the boiler tube cleaner/ descaler made by goodway for boilers. It is $17.00 a gallon and works very welland is very quick and safe.
Grainger's makes a nice kit to do it, all set up on a timer. The best test is to compare pressures going thru the coil versus bypassing the coil. As far as soap cleaning the coil out, it has been my experience that it does quite the opposite when the soapy water is flash heated in a coil. Our water has a very low mineral content and we run fresh water thru them so we rarely have to do this, if ever.