I actually noticed more of that a little sooner, my Yellows have tended to be pretty chubby though, haha. I would say the length started to slow down at about a year and a half. Yellows don't tend to be lengthy in the long run, so it's expected. I feed both of mine an xxl rat every 6 days, it's worked out fine. There isn't really an average size a yellow should be in a certain time span though, every snake will grow at it's own rate. You can usually tell if a snake is underweight, it will have a lot of loose skin and probably some bone showing through. I can't really tell you the average weight of an 8' yellow, because all I would really have to go by is mine, and I just don't have enough sources for it.
But my only suggestion would be to keep it on a more solid schedule, if it can take 3 large rats, then I would go ahead and just bump it up to a Xl (Jumbo) Rat, maybe keep it on a 7-8 day if you want to.
Hope that helps some, let me know if you need some more info!
Happy holidays to you as well!