At what size garden does a robot become a necessity?
A necessity? Pretty big. We have 1800m2 in total, although not all of that is going to get mowed, maybe 1000m2 or so?, so it makes a huge difference. But, personally, I'd get one for anything bigger than a postage stamp if I moved again. You can get smaller, cheaper robots designed for smaller gardens, although obviously not every space is going to be suitable but having the lawn constantly.
I estimate our garden to be around 100 sq/m, so really shouldn't take me long to mow, but as I don't own a mower currently, as we're only just about to seed for the first time, I'm in the market for equipment!
Even if it doesn't take all that long, you still need to do it. You still get a messy looking lawn while it's waiting to be done. And you still get the noise and disruption of a mower for everyone else. If you're the kind of person who likes doing garden then the robot free up your time to do the more fun stuff, and if you're not then it frees you from a tedious chore.
Plus, watching the little chap (we call ours the Grün Reaper) blunder around always make me smile