In this October 2003 article, he claims to have been advising Kerry for about 5 months. That would put his initial involvement back in May or June of 2003.
Wilson, former ambassador to the West African nation of Gabon, said he has long been a Kerry supporter and has contributed $2,000 to the campaign this year. He said he has been advising Kerry on foreign policy for about five months and will campaign for Kerry, including a trip to New Hampshire next month.Emphasis mine.