I know this is contraban here on the Kitfox forum, but here is a good picture of me landing my brand X on my runway (that I don't live near so I never cut it). You can't see my large Maule pneumatic tailwheel in the long grass, and my 850 tires are about at the half way point. So it is pretty long for most people. But landing in grass this long presented no problems whatsoever. In fact I think it is softer than a regular runway that is cut to a more normal 3 inch height. The only negative I noticed was getting weeds hung up on my tailwheel to rudder springs. (I have a new tailwheel now that doesn't use rudder springs so even that's not a problem now).
The takeoff was uneventful as well. I used less than 300 ft of the 2450 available without even trying. So if brand X does it, so will our Kitfoxes.
With all that said, I would avoid grass longer than this mostly because I'd be concerned about grass getting wrapped around things and causing some braking effect you might not want.
I used to fly my early Avid Flyer on the tall and narrow motorcyle tires out of any old 3 to 4 ft high hayfield and never had issues except for having to always clean grass wrapped around things. But the tires we all like to use today tend to be short and fat, so I don't think they would work as well in that long of a grass.