One of the many intercontinental flights I've been on. Not sure which flightlevel we're talking about, but I would think it's around the max cruise height of a lightly loaded Boeing 777 headed from Kuala Lumpur to Doha. We were only ~10 passengers, which allows for a higher and more efficient cruise.
EDIT: Max cruise altitude of a Boeing 777 is flight level 430, which it can only do if very lightly loaded. It is therefore probable that I've been at FL430 at least during parts of that particular flight. This corresponds to 43000ft, or ~13.1km