Funny you should bring it up. I am going through that right now. Although we didn't do it right this time, you can measure wear between the tail shaft and its tail bush using a poker gauge.
The poker gauge is usually provided by the original installer of the shaft and seal to the vessel, and kept by the Chief Engineer. I did find it eventually, one for the port side, one for the stb side, and it looks like a depth gauge, but specific to the vessel and within the box was a record of previous measurement. (Sorry I did not take a picture for you, I will try to when I get back). It fits into the the seal housing, see picture for the plug furthest left (I believe).
Because we didn't do the poker gauge method of measuring, the yard just but a bottle jack under the propeller boss, and a a dial indicator at the TDC of the tail shaft, then pumped up the shaft, then released it. The total movement equals the clearance in the bush. Ours was .064" on one shaft, and .030" on the other. The largest clearance indicated a problem, and when the shaft was pulled, the bearing was found wiped and damaged, see picture.
I am not sure what a typical wear is, but I wasn't too worried with 0.030" neither was the Wartsila Tech. I suppose .020" might be better though. I believe there is a rule of thumb, but can't remember it right this minute - 1 millimeter for every 100 millimeter of shaft diameter or something like that.
You can check out http://www.bv-industrie.de/brochures/
for more info on stern tube seals and bush, although I did not find anything on your specific question.
Hope that helps,