1% is the answer to your question.
I have 6 flights over 4 hours, plus a further 8 flights over 3 hours.
I have a few trips more than 4 hours.
Regarding bladder capacity I have though many times as a professional driver done the 4,5 h allowed.
I think most professional drivers do that.
Interesting thread. Out of my 446 flights (not counting 2018), none were above 4 hours. The closest one is 03:55 from Palma to Chambery. Only 8 were 3 hours and above, would have guessed that it should be higher.
I’ve done a couple of 4 hour flights in a Cessna 140. I’ve done 5.5 hours in a Piper Apache (this particular aircraft carried 7 hours of fuel) – Grand Canyon to somewhere in the Texas panhandle.
More than 3% of my flights were over 4 hours (flight time, no taxi). 17 on turbine, 5 on piston. Longest piston 4:59, longest turbine 5:59.
Zero percent. I believe my longest was 3-1/4 hrs, in a Luscombe which had about 6 hrs endurance. I typically enjoy 2 to 2-1/2 hr legs in anything, what varies is the distance covered in that time.
Longest flight : 2h48min. And only a handful above 2 hrs.
It was Lyon LFLY to Deauville LFRG in a DA40. With the autopilot, I could have flown 2 more hours 
As others said above, I only consider very long flight when I’m alone. And with no fuel totalizer, I am pretty conservative too.
I flew about 125 times (very conservative) greater than 4 hrs. As a percentage its too much work to rummage through 3 log books listing approx 3300 hrs. But Ive flown from the east coast of the US (NY) to the west coast Calif. & Az from NY to Jamaica, the Caribbean, to Panama To Canada and finally to Europe. Here my usual trips are 2-3 hrs. A really rough estimate is 4% of my flying is over 4 hrs.
Roughly 6% of my flights. The longest flight was 6:23 from Berlin to La Rochelle, followed by 5:22 to Figari, 5:13 to Calvi, 5:39 to Avignon, 5:33 to Guernsey, 5:18 to Dubrovnik and a lot more which are difficult to find in an old logbook.
20 out of 724 flights have been longer than 4 hours. (2,8%)
8 flights where longer than 5 hours and 6 flights longer than 6 hours.
My longest flight to date had a block time of 6:45 hours.
