Andy,
I replaced the glass in seven double pane, insulated windows (18" x
45") two years ago. Once you figure out how they come out and go back in it is easy and quick. The hard part is not breaking the stops that hold the glass in the window frame. Mine were plastic and about 20 years old. Several of them split and I had to manufacture wooden stops to replace them. The vendor I had make the glass couldn't get replacement stops.
Also make sure you measure the glass correctly. I removed the stops on one of the windows to make sure I had the measurements right.
The other issue is handling the 35" x 35" piece of glass. It's going to be heavy so wear gloves and go slow. All my windows were on the ground floor so that made it much easier. You should be in and out within an hour. The original glass is technically hazardous waste so disposal can be an issue. The vendor let me drop my old ones off at his shop.
Good luck. The view with the new glass was amazing as the originals were really fogged up.
dss