The Jargon File (version 4.4.7, 29 Dec 2003):

Ninety-Ninety Rule
 n.

    ?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.?


The Free On-line Dictionary of Computing (30 December 2018):

Ninety-Ninety Rule
90-90 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".

   An aphorism attributed to Tom Cargill of Bell Labs, and
   popularised 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.

   [Jargon File]

   (1995-07-14)