Is it ever measured for how long distance a front wheel has no contact with the tarmac on a superbike on a lap around the TT-course? Anyone seen a figure? My guess is about 10-15 % of the distance of one lap. What do you think?

It's going to be far less than 10-15 percent.

Just take any section of track and time it. Remember that 15 percent of one minute is 9 seconds, so for every 60 seconds of video you need 9 seconds of that wheel off the ground. It's going to be something more like 2 or 3 percent.

OK I'll take a look here: https://www.youtube.com/watch?v=ugZKd4mqvwQ