I've had some unlucky bounty questions(unlucky means I didn't find the answer), but now my reputation has gone to someone his answer didn't help, in fact it sometimes went to someone who has answered before I started the bounty.
what I'm talking about here is awarding the bounty to an answer when the time of the bounty is gone which seems unfair to me especially if there are people who just answer to have the reward regardless of the question.
Unfortunetly if we made bounty goes back to OP when an answer not accepted, then some OPs will play with bounties without awarding it to anyone, so I came with a solution, here it is:
when OP starts a bounty, it's not recursive but only when OP award an answer the answerer gets the reward, if no answer is rewarded and time has gone, the reputation set on the bounty is just gone, doesn't go back to OP and doesn't go to anyone.
I think this is the best solution, people now will really answer the question while bounty will still be not something to play, also I agree with the two-days time before bounty can be set and I agree with the 7-day time limit.
As some commented, sometimes OP doesn't care about reputation lost while a useful answer deserve it , so I don't know but maybe awarding an answer only when it is upvoted to some criteria would be more fair, for example , only answers higher than five votes get the reward because it is unlikely an answer like that is not useful(beleive me I had some bounty question awarded to only 1 scored answer and there was another answer which had the same score !)