My situation:
Shortly after XWW and I split, my youngest went off to college. My current (and final!) GF has a 28 yo who is pretty well launched. And my GF has been divorced for over a decade, so her daughter was not weirded out by our relationship, relieved more than anything.
So, we didn't have the full-on blended family challenges that would have made co-habitation very complicated. But we did have the stuck-in-our-ways situation to contend with, and the implied power struggle of having two established homes.
After 2 years together we gently started talking in the abstract about living together. No timelines, no geographies, just idle chatter. Her house is lovely and in a gorgeous in-town neighborhood in the city near me, but it couldn't support my side interests (loud music, woodworking). My house is very different - I live in the country in a beautiful custom home on three wooded acres. It is situated on a ridge overlooking a gorgeous environmentally preserved river. It's truly breathtaking. It has a detached music studio and a nice basement woodshop and at last count, some 50+ windows. It's bigger than her house too, but nonetheless cozy. But it was the marital home for me and what's-her-name, so that was a big vote against my joint.
So, it seemed we were destined for a new to both of us house. But the more we casually looked at real estate, the more dissatisfied we were with the offerings. So, she came around to the idea of moving into my house before I began to sell the idea. I was ecstatic.
Now, over three years later, we are redoing aspects of the house (kitchen countertops, re-painting, hardwood floor refinishing, tile tubs in place of cheap acrylic inserts). And she has reviewed every item in the house for "Her Cooties" and subsequent purge.
I suggested she rent her house furnished rather than sell it. 1) It will let us shuffle the furniture around to keep combined good stuff with us and more rental worthy stuff at her house. 2) She lives two blocks from a major university, so it's prime for sabbatical home renting. and 3) I joke that if she finds out I'm an asshole, she just needs to wait until the end of the semester. She already owns two rental houses, so she's already got a rolodex of plumbers, electricians, etc... at her disposal.
I also co-habbed with her for 4 months when I let my daughter and her family move into my house as a staging process for her moving back to the area. And I must say, our domestic rhythms really synced.
She has her house rented starting the end of December, so the deadline is there. She has lots of house purging to do, I have lots of upfitting and oversight of the upgrade process, but I am super excited.
The remaining challenge is to make sure her friends feel comfortable making the 15-20 minute drive to our bucolic paradise. They're great people and I will do everything to make them feel welcome.
Our situation is someone unique, without young kids in the mix and having a fair amount of parity in our situations.
Why not marriage? Well, I don't have a great track record, so I want to wait a bit and make sure we like each other's morning breath for months at a time.