The function persistence takes a positive parameter integer and returns its multiplicative persistence, the number of times its digits are multiplied until a single digit is reached.
For example (Input --> Output):
39 --> 3 (because 39 = 27, 27 = 14, 1*4 = 4 and 4 has only one digit)
999 --> 4 (because 999 = 729, 729 = 126, 126 = 12, and finally 1*2 = 2)
4 --> 0 (because 4 is already a one-digit number)