fwiw I haven't really noticed much change due to 'burn in' with my pair, with the hours I've put on them at the office. I will say they sound a lot better with the ipod than the Transit. I never realized how bad the transit sounds until getting a second unit recently.
Anyway, I think you can expect them to sound at least reasonably close to what you're hearing now, unless something magical happens after some # of hundreds of hours.