I do a complete wipedown of the heli after every flying session. So the external engine surface is just quickly cleaned with fantastik or 409. Internal engine cleaning? Never. I run synthetic fuel so even engines run for years are sparkling clean inside. The only time the engine internals are cleaned is after a crash to remove any possible dirt that was ingested.
If you run castor oil fuel I would recommend a thorough cleaning every 3 months if you fly a lot, 6 months for weekend pilots.
Every year the bearings and O rings are inspected.

Fritz