The 35-year-old, who joined the club from Newcastle United in 2009, recently parted ways with the Cottagers following the end of his contract.
Despite admitting that he is sad to be leaving Craven Cottage, Duff claims that the most enjoyable years of his career have been spent there.
"I think it was the right time for everyone, for me and the club, to part ways," he told Fulham's official website. "It's sad, but that's the way it is.
"I've matured as a person and a player at Fulham. Since I've been here I've got married, I've had two kids, so it's been a special time in my life and I stand by what I've said before that my most enjoyable time in football has been at Fulham."
Duff recently suggested that he could spend the next part of his career in Australia or the USA.