I am looking for a ballpark estimate as to how many miles I have left on this rear tire:
Do you get a lot of punctures while riding? I got a puncture every month or two while commuting. When that suddenly increased to a weekly puncture, it becomes time for new tyres. It was 80% the rear wheel.
I can still see the central tread line in the middle of your tyre, so it looks perfectly fine to keep riding IMO. Also, its probably got a little faster as the central knobs wear off, assuming you ride smoother roads.
If you're riding off-road aggressively (ie competitively), then the lack of central tread may become more important.
When punctures get more common suddenly, then replace both tyres.
Estimate: 500-1500 km, (300-900 miles) I'd certainly ride that tyre.