As for myself, I hit plateaus when I was 21k on the old KGS ranking system (over 2 months; this was super long considering I was improving from 25k to 22k in less than a week and it wasn't until I was SDK or so before it would take months at a time to improve a stone). Another point was when I was 5k (also on the old KGS ranking system); this was at a point in my life where I lost almost all interest in Go and was on the verge of quitting - until I went to Go'n'Games for the summer of 2006. Not only did I improve 4-5 stones in just 2 months (I wish I could have saved a graph on my old KGS account to show the progress) but my passion for Go returned and I was hooked again...
...until college kicked in. This was when I was around AGA 3d/KGS 2d or so. The plateau lasted for about 6 months, then I slowly started improving again by fitting in time to do tsumego and play more games with classes (this nearly slaughtered my GPA). Slowly crawled up to AGA 5d/KGS 4d and stuck since (it's been about 2 years now), though recently a few of my anon KGS accounts have upgraded to 5d, though they haven't had a chance to weather through too many games just yet.
I think I've reached that point where I'm kind of "settled" about it. I still enjoy Go, but nowadays I just lack the time and the passion to continue improving again - I hope to change this after I'm done with college...yeah right
So what about you?
