There should be an option to set the default poll length in ACP

When polls are created, the default option for poll length is 0 a never ending poll. 

Based on my experience, most users either don't see that value of disregard it. The end result is most polls are endless, while typically the poll question is inspired by recent events. 

I would propose there should be an ACP setting to allow the default poll length to be specified by the board administrator.