First of all, I suggest you to go for the short ones. But even the short ones sometimes become longer for different reasons: hardcap not reached, bad market conditions and so on. Eventually the time is not the defining characteristic of a good bounty campaign.
In my opinion, a good bounty campaign is for a project that really deserves attention and that will really do well.
One such project is #Blocnation. They have this awesome concept of tokenising the payment inside a private event. Music festival, bracelet, contact-less, pay instantly with BNTN, your drink has been payed. The team has been around with this product for some time now and have had great success. More success to come though.
This is a kind of project that I would support: one that I like, that has great potential and is trustworthy.
Hope I made myself clear.
Stay safe!