You have no choice of frequency, its fixed by the period of the mains, and even the term PWM is almost meaningless, you can drive, 1 cycle in 100 up to 100 cycles in 100, or 1000, or 10,000 - a lot depends on the thermal capacity of your load. A big bulky furnace is unlikely to have a timeconstant less than minutes, a tiny resistor a time constant of a second or so, so your control band cannot really be more than 1 seconds worth of cycles.