I'd be surprised if you can fined any definitive numbers for 'Long Haul', it's more a state of mind! A crew could be sitting around the pool in Sydney having arrived from London via a series of six hour sectors and several days, to them it is still Long Haul.
Generally accepted sector times, after the advent of the B747-400, were 0-4hrs Short, 4-8hrs Medium, 8-12 Long and 12+ Ultra Long Haul.
Please post if you find the definition published by any authority.