It's hard to put a specific number on it, since it's been progressively improving the whole time I had it. I bought it in the fall of 2023, and got it "running" then, but it was not actually doing any useful work. Summer of 2024 it was doing useful work but required constant hand holding and a series of hardware and software upgrades to get it more stable. This year it is actually doing the majority of the mowing. It still requires frequent rescue but it's a lot less work than actually mowing the lawn myself.
However my lawn is probably the most difficult lawn of any openmower user. I'm in Vermont, so I have very steep terrain, a bumpy yard, poor GPS reception, very wet weather, and also my lawn is very large and complex shaped. For a simple use case it would be working great a long time ago. I also chose to do a "Mowgli" build which is based on more reverse engineering, which added complexity and unreliability (but saved some money).
How much time did it take you from the moment you started the project to the point where it fell like it was up & running?