Common with self-employed geeks and students.
Comparible to the finite number of times you can fold a piece of paper, the wanker's dozen is rarely beaten.
Derived from bakers dozen, except the bread rolls are spooge-filled tissues
M: "My bell end is red-raw."
J: "Why's that?"
M: "Wanker's Dozen"
J: "I am Not suprised"