I usually get 900+ miles from trainers. Wear lightweight trainers like Asics Gel Lyte, Reebok Premier Lite. Do about 50/50 roads/trails. I use flats for racing (1 mile to marathon, trail races, triathlon) and an occasional speed workout. I prefer to use them on race day only for the mental edge of putting on the race gear.
In my experience I have gotten anywhere from 75 miles to 400+ from flats, but usually 250-300 miles. The 75 was a pair of Reebok Racer X's that may have been defective. I went through the toe. Sent them back and got a replacement pair that is going strong after about 200 miles. The longest lasting flats I had were a pair of the old Nike Air Socks (the blue one with laces, not the yellow one with buckles) and a pair of the Nike Duellists. In fact, the Duellists I have are from about 1992 and I still do a race or two a year in them.