The rule of three

Here's a guideline Don Roberts gave me: The first time you do something, you just do it. The second time you do something similar, you wince at the duplication, but you do the duplicate thing anyway. The third time you do something similar, you refactor.

Or for those who likes baseball: Three strikes, then you refactor.

Martin Fowler, "Principles in Refactoring", in Refactoring: Improving the Design of Existing Code (2nd Edition), 50.