A reasonably good measure of trending any HP degradation is available to the operator of a fixed-pitch prop aircraft, and that is to keep a log of static runup RPM at home base. Record the OAT, wind, and QFE along with RPM and you'll get a good picture of what's going on. Every 50 hours might be a good interval for these checks, at least until you have a good database.
Unfortunately a variable (constant-speed) prop complicates the matter, unless there's a torquemeter like the R-2800 has.