Michael,
Depends on whether you're talking about sanding with or against the grain. Sanding against the grain (i.e. with the lathe on), I sand for progressively less time as I work up through the higher grits, whether its sandpaper/Abranet or Micromesh. You may find that when you turn the lathe off and sand with the grain, you'll need to spend more time with the 1500 grit MM than the others, because of the way MM is made. It has a more even particle distribution, which makes it less likely to leave a big honkin' scratch in the finish, so once you've used it to sand out the other scratches, it gets easier. Plus, in my (admittedly limited) experience, as you get to the higher MM grits, all you're doing is putting the cherry on top of the icecream sundae - if you press too hard, or sand too long, you'll work through all the effort from the previous grits. Granted, it will take you a while to go deep into the wood with the finest grit MM, but still, I don't like to leave the MM on the blank for TOO long.
So, after the long-winded answer, my short answer is that I spend about 30-45 seconds at the larger grit levels, and in some cases 10-15 seconds with the smaller grit levels.
Again, this is my experience; I am curious to see what the others say.