What is a good rule of thumb to produce efficient heating?

Balance between the size of the part and the depth of power

Usual rule of thumb is to have a time cycle that is 25 times the square of the diameter of the part for efficient heating. So if a part is 2 inches in diameter, the heat time should be 25 times 4 which is equal to 100 seconds. If faster times are required then more than one part should be heated at the same time.