Every day, unless I didn't sleep well. Those times, I tend to wake up and fall asleep a half dozen times or so, which means that by the time I'm properly awake enough to notice tumesence, it isn't there any more. Can't call it morning wood if it went away an hour or two beetle before you actually wake up.
But, damn; some days that's what wakes me up. I had kinda hoped that part would stop as I aged, but I'm 50 and it's still like that at least a few days a month.
Back in my younger days, like you, it either wasn't every day, or the last cycle of it happened well before I was aware of it. I can't recall the exact frequency, but it's one of those things where it happens multiple times a night during sleep, and whether or not it's morning wood is a matter of timing.