I would say at least once a fortnight simply because our road is dirt with cow shit and any traffic turns up huge amounts of dust. Both bikes are housed in my big workshop and fine steel linasher dust settles on most thing so I keep that off. They get cleaned before any long ride and within a day or so of getting home. Perhaps 4000km between washes in that scenario.
One of these bikes I bought new and often riders comment on the new bike only to discover its 6 years old and over the magic 100,000km mark.
So I think it pays off in my case.