Martin,
I have had my mono extremes for a little over 5 months now the change is a gradual one. They start off sounding lean and over time you just seem to say "wow" that's starting to sound really, really good. And yes I would tend to agree with the 3 month break-in estimate even after that they still seem to keep evolving to some degree. Although that just might be me as I do travel a fair bit on business and coming home after a week or two on the road the system sounds better than it did before I left.
One tip "leave them on" it does take 2~3 days for them to charge back up again if you turn them off (seems silly I know but it's true).
Good luck and good listening you're going to really enjoy your mono's!
Regards,
Fred