After spending 2 hours crammed into an ET seat* my answer is: Zero hours.
WT I've found not bad actually, mainly as it's always been quiet on my flights (lucky, I know) so I've had 2 or 3 seats to myself. Can't imagine it being much fun for a 10 hour flight with just the one seat.
*(extra legroom according to the website, looked the same as others to me. 8E on the A321)