I did a manual clean on mine with a set of rifle cleaning brushes a month or two ago. My car has done about 85k, with me doing the last 25k of them modified. My valves had hardly any carbon buildup on them at all. I don't know if it was a combo of the fact I drive my car hard, have an OCC, occasionally run meth and have had a new valve cover fitted recently. I heard a 135i race car had a valve cleaning done on the other side of the forum and despite average mileage, the valves were pretty clean.
The process didn't take too long because they weren't too bad and honestly speaking, it felt exactly the same and it still has a very slight stutter on idle.
Oh and I managed to completely ruin my hoover in the process.