In terms of just flat-out good propagation, I've been able to pick up detectors 50 miles Northwest of Toronto, and trains and dispatchers at least 100+ miles in the same direction. To the East, I've heard trains calling signals as far as Utica, and to the West, I've heard trains calling all the way to the PA/OH border on CSX (the best I've done with detectors in that direction is Ripley, right by the NY/PA state line, and I've managed to pick up a dispatcher West of Deshler, OH). I've also had a few nights where dispatchers in Detroit come in nice and clear, as well as the old B&O main. All this on my base radio here, which is the Rochester, NY feed on railroadradio.net (Icom 2200H and a Traintenna mounted at 30' out in the middle of farmland).
Now, if you're talking actual skip, I couldn't tell you. I listen to my radio when I'm home, but being a college student, working, railfanning, etc., I really don't monitor it that much (plus I can't get any programs to actually work that will record all the transmissions due to some hardware issues on the server). The best I've done in my car (tuned 5/8 wave and another Icom 2200H) was picking up the NS detector in Portage, PA on the West Slope from CP-382 as though it was being transmitted from right next to me (actually, I picked it up both the first and the second time...I believe a train was in there too, but the whole thing only lasted maybe 20 seconds or so).