question of week

1..we have prime number 11, now add its digits in it e.g 11+1+1=13, which is again prime number.again 13+1+3=17,we get prime number.repeating same with 17, we get 17+1+7=25, which is composite number.we complete
d three steps to obtain composite number. tell, how many maximum number of steps possible for a prime to get a composite number.