My stats are 425,000 Mdots, or just over this really, and the head was at 770 days, or just over this. I took a pic of the screen, since the head was acting up, and sure enough, within a couple of weeks, it had to be replaced. The other head went soon after, but I think this one was just below 400,000 Mdots. Time wise I guess they lasted pretty good. I clearly am not printing as much as bill t., but i try and make sure to do a print every 2 days so that it never sits for too long.
As bill also states, you can tell its about to die because of the constant cleanings and error messages. I was a little pissed because it randomly started dumping ink all by itself without me initiating the cycle. Would have been nice for it to just say the head needs to be replaced and leave it at that. If I didn't have one handy, I'm not sure what I would have done because after it dumps the ink, the head moves to the centre, so there is so no way to put it into sleep mode at this point. Its also a bad idea to have a new head sitting around for months because Canon will only warranty from the date of purchase, from what I read here, as opposed to date of installation.