Kyman...let me see...I started thinking about it in 2006 with the episode "Man Bear Pig" when they were trapped in the cave and I think..thats really when it started to become a running gag in the show itself.
Before that, I shipped Eric with Butters and didnt really give him a whole lot of thought.
Thhheeeen "Imagination Land" and then the Tourette episode was where I started shipping it-
Then "Tonsil Trouble" came out like the very next season and I was like. OKAY. THEY WERE MADE FOR EACH OTHER. And then I officially loved it more than Eric/Butters...and it was the Kyman ship that really kinda had me give Eric's character a second look and how he ended up being one of my favorites and then making Kyman one of all time favorites!
Sooo I guess I am gonna say since 2006-2007 which would be...about 7 years. Not that far behind Stenny.