I've never really been bothered by airsickness, even during a Yak52 aerobatic flight (where I instructed the pilot to "go bananas"!). However, the other week, I'd just come back from a hour of my own local flight with extensive airwork when my instructor invited me to go with him straight away on his recency flight. It was the longest I'd been in a light aircraft at two hours - with lots of steep turns, stalls, circuits, etc - and started to feel a little queasy on the inbound run. :rolleyes:
I put it down to never going past the 1.3 hour mark before - plus, it wasn't two hours of straight and level! Haven't had a problem since, though.