Wikipedia & Chronicle at the time in 2017 both say 3 years
Wikipedia
After Southampton released Isgrove at the end of the 2016–17 season, he signed a three-year contract with Championship club Barnsley
Chronicle July 2017
Promotion winning winger Lloyd Isgrove has returned to Barnsley on a free transfer.
Isgrove, 24, was a free agent after leaving boyhood club Southampton at the end of the season. He has signed a three-year contract with Barnsley.
Now I make that until summer 2020