I'd estimate that the Eagle F1s on the back of mine have done around 17,000 miles since I got them in August 2014 and I'm now considering getting them changed, I'd say they're on 3mm now. Considering at the time I got the Eagle F1s I was reading they didn't last as well as other tyres I've been quite impressed with them and they still seem to grip quite well.
I'd potentially try out ContiSportContact 5s, but my step-dad had those on the back of his E46 touring and they lasted him no more than 12 months doing less mileage than me, and they were on (more than likely just below knowing him) the limit when he changed them.
Generally 2-3mm is when I get them changed.