If you say duration doesn't matter,you could be right but in my opinion I think short duration bounties pay more most times, reason is as short as a bounty is,before more people will know about it,the bounty is already ended and the reward pool will be shared among those few that registerd but in long, as the day go by,more people tend to register which at the end help reduce the bounty pool reward because more people to share from the reward pool.