I find 8 hours a breeze ............ just add another 6 hours to how you felt after your longest trip to give you some idea

. I guess it depends on your tolerance levels and often the length of your legs versus the available leg room, particularly if you travel economy.
I haven't flown Air Philippines, been with BA, Emirates, Cathay Pacific and KLM, The last time I went in 2010 (unplanned urgent trip) it was Air France Heathrow to Paris, Paris to Hong Kong with a 5 hour connection wait and then Cathay Pacific Hong Kong to Manila. It took 27 hours and I didn't sleep a wink!

I'm not tall but being cramped in the same seat for such a long time is a killer. I have also been fortunate to fly business class which certainly helps to make the journey a bit more bearable but I'd prefer to take a magic carpet any day!
