Quote from [How does the bounty system work?](http://meta.stackoverflow.com/a/16067/232872):

> ### How do I start a bounty? When can I start a bounty?
> 
> A bounty can be started on a question two days after the question was
> asked.
> 
> To start a bounty, click on the <kbd>start a bounty</kbd> link at the
> bottom of an eligible question.  The bounty panel will open.  Use the
> dropdown to allocate the bounty amount -- anywhere between 50 and 500
> reputation, in 50 point increments.
> 
> You must have at least 75 reputation to start a bounty, and at least
> as much reputation as the bounty amount.
> 
> The bounty award will be subtracted from your reputation when the
> bounty is *started*, not when it is awarded.