My first trip was in 1979. So that would be 27 years. This will be my 12th trip and my DH's 6th though. Since 1998 I've only missed 2 years. I had each of my DS's the years we missed. Don't plan on missing anymore.
18 years for me - there were periods where I would go as long as 5 years in between visits, but have been going at least annually since 1996. There will be 3 trips in total for the 2006 year.