It depends on your ranking in the bitcoin conversation account, and the lower the ranking, the lower the bet.
The longer the bounty ends, the less likely you are to receive a reward, because as the bounty extends, the more participants there will be, and the more people will share the token.
Yes the longer the bounty, the more participants there will be and more people will share the token and thats what it makes to get a fewer earnings with longer bounty, that is the negative side of it.