your answer is here: Frequently Asked Questions
There are a few other rules around bounties:
•Questions must be at least 2 days old to be eligible for a bounty.
•Users must have at least 75 reputation to offer a bounty.
•There can only be 1 active bounty per question and per user at any
•Once initiated, the bounty period lasts 7 days.
•After starting a bounty, you must wait 1 day before awarding it.
•If you do not award your bounty within 7 days, the highest voted
answer created after the bounty started with at least 2 upvotes will
be awarded half the bounty amount.