does the following route is doable ... Paris - Bangkok ... with 290 pax ?
... if you stop somewhere midway to refuel
Otherwise would hazard a guess that it is just not possible, even with Winglets. To many Pax. If you go down to 260 pax ... it might work.
Naturally the main challenge is coming back where you are going against the wind.
The longest sectors I have done were Beijing to Kiev, and Kiev to JFK. 250 seats, no winglets. It was around 9.5 to 10 hours.
As the previous poster mentioned, after 8 or 9 hours you have to start trade payload for fuel.
Some "ball park" numbers:
Aircraft Empty = 90 tons
MTOW = 185 Tons
290 Paxs = 30 Tons (being conservative)
Fuel = 65 Tons
65 Tons of Fuel is good for around 11 hours (being conservative). Bangkok to Paris is likely to be longer than 11 hours ...