Most of the bounty programs won't mention this rule in their thread. But when users want to leave the campaign in the middle, they will impose this rule.
So far every bounty I participated had rule about it and I don't recall that any bounty which didn't let me leave if rule about leaving wasn't there.
You can't permanently stick to a campaign which won't end after a certain period of time. I have seen some campaigns which are running for almost a year. Do you still think it's a good idea to stick into these type of campaigns?
Don't do long term campaigns, stay some time but not too long, manage your time better and join each month different campaign.