REALLY depends. If hes moving out in the open, yeah, it should be about the old 1.5k (the icon means you don't actually have to do any of the work, so no further than that).
However, if he moving up under some cover...... 600 yds.
If hes stopped in cover, just about any cover at all..... you should have about 400yds worth of warning, and thats only for game-play purposes.
Personally, I would love to see a line of sight-style icon range. Say there has to be at least 40% of the tank clearly exposed for the icon to appear (since the icon is a MASSIVE advantage that they didn't have in real life).