code question

I just took the Mass. State Oil Technicians exam and I was asked about the amount of flexible metal sheathing that was allowed in the burner circuit. I have not seen this in the code book or was it brought up in the classes I took before the exam. The possible answers were 6 feet, 10 feet, 36 inches and unlimited. Does anyone know the correct answer. I can't find it anywhere. Thanks.   Ed