I'm seeing the same thing you are--tickets seem purchasable for the next three months. As of now. Ticket-release policies can change without advance notification, though I have no reason to think St. Peter's will be doing something different later this year.
I see no sign that St. Peter's entry tickets sell out instantly. They apparently have allocated a fairly generous number of entry slots to online sales. You can see how good availability is right after Easter, which is less than two weeks from now. I suspect many travelers don't yet know entry times can be reserved. That's a fairly new option, probably not reflected in printed guidebooks.
What I'd do is check the St. Peter's website at least weekly from now until about mid-May (four months out), and then ramp up to perhaps twice a week. By "check the website" I mean drilling all the way down to the list of entry times, because a date will appear to be available if even one entry-time slot has enough openings to match the number of entries you've requested. It's not unusual to see entries for popular sights available only for very late in the day, with all the earlier entry times sold out. Very late entry might not allow enough time for a good visit.
I'm confident my advice is serious overkill, but I don't want you coming after me if something truly strange happens with the sales policy and customer demand. When a city has multiple sights requiring time-specific tickets or reservations, as Rome does, you do have to be careful not to get boxed in.