On Fri, 2 Dec 2011 17:24:36 -0800 (PST), Bill D
wrote:

What is the generally accepted service life of the "donuts"?

Long.
We changed ours in 2009 when our 21 had about 4.000 hrs and 12.000
flights (it was built in 1986). All landings on a less-than-perfect
grass airfield.
We changed them when the gear started to rattle because the donuts had
become permanently compressed.


Andreas