yep the longer is bounty and the more people join the less profit, but there is exceptions if manager who check will make 90% red.
Of cause there are a lot of factors, which influence earings from long bounty.
As you told, it is important how strict bounty managers and bounty rules, if it is strict a lot of participants will be deeted from bounty.
Also pool and contributons are important (big pool + big ico contributions = good bounty earnings).