How far out are aircraft types/seating charts finalized (with the exception of further swaps)?

I booked a trip where the BOS-EWR leg was showing a 737-800, but had the hybrid placeholder seatmap. Since this route is almost always a 738 or 739, I figured it wouldn't be a big deal.

Looking at it today, it looks like it is now an A320 which is uncommon for this sCO route.

The flight is in July. I'm hoping they swap it again.