Had a bi-lateral pneumothorax 15 years ago at 31.Had the pleurodesis op done. Been on 3 hours flights before. Is there a danger on long haul flights?

You should be ok. Spontaneous pneumothorax occurs when a congenital bleb ruptures, leaking air into the space (pleural cavity) between the lung and rib cage. Pleurodesis is a procedure in which the pleural space is artificially obliterated by inducing adhesion formation between the lung lining and the rib cage lining. There is no evidence that air travel precipitates recurrence of a pneumothorax.
Probably not. If you had pleurodesis at that time and no recurrence it is probably ok but it depends on what caused the bilateral pneumothoraces in the first place.
Very Low Risk. The pleurodesis should have scarred your lung to the chest wall and prevent it from collapsing as it did before.
I don't think so. 1. You had pleurodesis, so chances of recurrence are low; 2 you had no problems for 15 years, so it is unlikely to recur now, especially after 3 previous short flights; 3. It is probably change is pressure rather than length of flight which can trigger pneumothorax, so length of flight is probably not as important as the number of takeoffs and landings. Ask a lung specialist to be sure.