You can set the bounty whenever for whatever reason; they're highly personal things, like voting. We're not going to moderate it or tell you how to use it unless you're causing trouble which is unlikely. I say this to emphasize it's up to you, there's no correct behaviour here, and you won't get in trouble for doing something inappropriate.
If you post a bounty asking for things to be updated, and they're already up to date, the most damage you've done is spent some rep and time and energy that you didn't have to spend. For that reason it's generally in your favour to make sure you'll actually be getting something out of it.
It also helps to understand the size of the of undertaking you're asking for. What if you've actually asked for an enormous task, but only posted 50 rep for the bounty? Will people really respond? Or what if it's a trivial task, but you overestimated it and posted 500 rep for that task? The bounty should be proportionate to what you think the work will be like. Our estimates of how much work's involved may not pan out as accurate, but they're estimates, not exactimates.