ANSWER
203 years.
Twelve amendments were proposed to Congress on September 25, 1789. Numbers 3-12 were approved, and became the first ten amendments, collectively known as the Bill of Rights. Number 1, related to congressional apportionment, has since become irrelevant. Number 2 was initially ratified by six states, but fell short of the necessary approval by three fourths of the states.
In those days, there was no time limit on proposed amendments, so it has remained through the years, mostly forgotten, until it was rediscovered in 1982 by Gregory Watson, a student at the University of Texas. Watson wrote a paper on the subject, recommending that the amendment be adopted by the rest of the states. His professor gave him a "C". Unfazed, Watson began a one-man campaign to get the amendment approved, for years spending his own money, and working tirelessly on it nights and weekends. It worked -- on May 7, 1992, Michigan became the 38th state to ratify it, making it the 27th amendment to the US Constitution.
In case you are wondering, here is the full text of the 27th amendment:
"No law varying the compensation for the services of the Senators and Representatives shall take effect, until an election of Representatives shall have intervened."
For more information, one of the best sources is the following article by John Dean (yes, that John Dean)... http://writ.news.findlaw.com/dean/20020927.html
WHO GOT IT RIGHT: Marc Quinlivan, Bob Milligan, Robin Campbell, Marika Thiessen, JP Weigt, Adrian Beckett, Paul Delson, Luz Velasco, Yanitz Grinell, and Jim Ahumada.
