Sacha was mentioning that a certain large corporation's consultants aim for something like 92% utilisation (I may have the number slightly wrong, but it was in that area), that is, in an 8 hour day they want 7 hours and 22 minutes of that billable.

I would think at that point you have to have a significant support infrastructure, after all, just answering intra-company email would probably kill that ratio, as would keeping track of your utilisation rate; you'd have to have someone running interference on internal matters to make it remotely possible. Of course, they probably have broader rules for charging clients, i.e. they'll charge you for a whole day if they're on-project for more than X% of the day, rather than charging by the minute.

I currently aim for 5 hours (62.5%) in my eight hour day, though I often have to work significantly longer hours to get up to 5 hours billable. Today, however, I was pretty much completely on-task and on-track, hitting 6 hours and 25 minutes billable in a bit over eight hours.

Key to that was that there were two development tasks, rather than being all maintenance and troubleshooting. I can zone out and work solidly for hours on a development task, but the broken-up tasks have too many points at which I can lose focus and have to log off billable. Of course, zoned-out work is pretty hard on one's body (don't take breaks or stretch or whatever).

Anyway, I don't think I'll increase my target, despite the possibility that I may fall behind the Joneses :) . Sure, I can hit 6 hours on many days, but it just doesn't seem necessary... 5 hours/day produces plenty of income even at my rather low rates and it means that I don't spend the rest of the day in pain.