Is there any documentation that has the approximate rate of speed

The heating rate at a particular infinite control setting varies from one kiln to another. This is because the rate at each setting depends on the size of the kiln, amperage, type of insulation, etc. Another factor is the age of the switch. You can use a pyrometer to figure the firing rate at each setting on your particular kiln. The rate will be fairly repeatable. However, if you change the switch, you will need to figure the rates for the new switch too.