Does anyone keep track of the longest acceptance speech at the Oscars?
The folks at Guinness World Records do. According to them, Greer Garson spoke for five minutes and 30 seconds after accepting the award for Best Actress for her role in Mrs. Miniver in 1942.
Here's some more information about the 1942 Academy Awards.
As an aside, while she didn't take as much time, Olivia de Havilland thanked a record 27 people in her 1947 acceptance speech.