As a new owner...
I was able to make a 4th of July reservation for this year at 9 months out. The following week became available at the end of February - a lot of inventory showed up for all unit types, from somewhere in June (didn't check) until end of July. Inventory was gone within 2 days or even less; I checked just out of curiosity.
Thanksgiving for this year was easy, 9 months out (did not book it, but was curious) I think I saw it for a week or so.
I was also able to reserve this week 12 at the Valdoro, I don't recall exactly when I saw it, but it was 6 weeks or so before. First there was a lot of inventory for the month of February (we weren't sure when to go) except for Presidents Week, and then a few weeks or so later, the whole month of March showed availability. And, I could even have reserved some of these weeks through RCI, using less points.
Almost forgot - we were considering going to HHV during Easter this year and that showed up somewhere in October - again, all weeks in February (including Presidents Week), March and April, all unit types. Presidents Week and Easter were gone within 1 or 2 days, the rest was there for a longer period.
I guess I have been very lucky but so far I did not have any real problems with getting what I want. But then again, I guess I can forget Valdoro weeks 51/52.
Maybe we should have a sightings board for HGVC?
I do recall that there was a mention that they were going to make some changes because many owners do not want to book as far as 9 months out. Not sure though.