Therapy has varied a lot among us. Some doctors don't even believe in sending their fusion patients for PT. My therapist had me work on isolating and strengthening my core muscles at first, then added some leg strengthening exercises later. Other therapists have done a lot of pool work --- good for us back patients --- or typical PT exercises that were maybe not really the best thing for a fusion patient. When it's time to go to therapy, try to find someone who has had lots of experience with back patients.
There have been people here who have gone back to work as quickly as 2 weeks because they had no choice. They were not happy campers! 12 weeks out of work is about average. You'll be tired and maybe still on pain meRAB at that point, though maybe not. You'll want to go home and just rest as soon as work is done. But you'll get through it.
If for some reason you don't fuse, they can go back in and do a revision. Of course, that means you'll be laid up again. This is a big surgery with a long recovery time, so you'll want to do everything you can to help it "take." That means giving up cigarettes if you're a smoker, rest and walk, rest and walk, rest and walk everyday (walking gets blood circulating around your spine and is essential to get bone growth going), and maybe ask your doctor about a bone growth stimulator. Also, many doctors use BMP, which is a synthetic bone protein material that helps get bone growth going.
And don't worry about me. I sometimes fall into thinking "poor me," but I have to count my blessings. I'm a lot better off than many people!

But I appreciate your empathy! No one but the people here really understand what it's like! It's nice to know I have a place to vent on the hard days!
:wave: Emily