What is the recommended time frame for a bounty campaigns?
Each campaign has its own time frame. There is no one single term. On the timing of the campaign bounty written in the rules.
what you say is very true but from the many prizes available, most of them are 2-3 months but there are also more. sometimes there are also gifts that extend the duration. so there is indeed no certainty of duration for Bounty. but I suggest to follow the Bounty which is not too long.