verticalflight
There does seem to be a paucity of information out there about download frequency. I think CAP693 is no longer valide since the AAD was withdrawn. There is CAP 753, but even that does not give much info on dowload frequency, however it does mention that the system should ideally be able to support rotors-running downloads. There is not much in EASA land, that is because EASA retain a degree of ludditism for HUMS. In FAA land there is something called AC29-2c-MG15 but I couldn't find it.
But there can be no doubt that best practice is to download every time the aircraft visits a base where there are download facilities. Just like the fuel in the bowser and the runway behind you, there is little more useless than the HUMS data still on the card.
HC