Having done a lot of roof down euro touring in the aero I find that 2 hour stints with short breaks (essential for refuelling anyway) works well. More than 10 hours on the road tends to be too much as after that your reactions are not so sharp.

Heading to switzerland my routine is usually something like

Night before - head to chunnel for 8pm over the chunnel and stop around rouen / reims.
Day 1 early start and drive to destination.

That means its only 1 day spent on the road, the bonus being that the speed cops are in bed and the roads are clear around calais during the night before section..

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Maxxed out!