I'm 39 and my husband is 38, married 17 years; no kids.
My preference would be about every other day to once every 2-3 days, depending. Once in awhile, I want an orgy of indulgence; occasionally my drive takes a fall and once a week would be fine. *With* mutual desire and alternating initiation.
Happily for me, that's *about* what I'm getting at the moment; perhaps just a tad under. I'm not complaining.
So call desire = 14 times a month; probably 10-14 in practice.
"Show me a completely smooth operation and I'll show you someone who's covering mistakes. Real boats rock." -- Frank Herbert