Ninety-Ninety Rule

"The first 90% of the code accounts for the first 90% of the development time. The remaining 10% of the code accounts for the other 90% of the development time.” Attributed to Tom Cargill of Bell Labs, and popularized by Jon Bentley's September 1985 Bumper-Sticker Computer Science column in Communications of the ACM. It was there called the “Rule of Credibility”, a name which seems not to have stuck. Other maxims in the same vein include the law attributed to the early British computer scientist Douglas Hartree: “The time from now until the completion of the project tends to become constant.”

http://catb.org/~esr/jargon/html/N/Ninety-Ninety-Rule.html