Snap i looked at mine 2 weeks ago after about 4k since I last looked. 3k of what id call urban driving and 1k upto scotland and back on the motor ways. Mine had a bit more then yours maybe 1.5-2mm of soot. But I put that down to my increasing amount of urban driving and not having much time on the motor way to get it hot and give it a good italian tune to get it cleaned. SO as you say every 10k or 12months would be reasonable for a clean. But if you do nothing but town driving then Possibly look at cleaning at any point in the 6-10k miles range.