Hi Mr. Sudeer.
To be very honest there is no hard and fast rule to determine the length of kiln burning zone. However you need to identify your overall BZ with hard regular/normal coating. A tentative rule of 5 to 6 times effective kiln dia. is considered as burning zone. eg; if your kiln internal dia. is about 4m, your approximate BZ length will be about 20-24 meters.