Copying my comment from gerrit:
The duration of blocks can be configured both on a per-filter basis and via globals. But that's just because of historical reasons: previously, it was only configurable per-wiki, then we added per-filter customization, and the global setting became just the default value for the dropdown.
However, for other actions, I'd like to stick to a single config type: either per-wiki or per-filter. Having both is IMHO hard to maintain, and not much useful. Hence, I think we should discuss (on phab) the pros and cons of both approaches and determine which one we want for blockautopromote.
Given the nature of the 'blockautopromote' action, I think users won't need to customize the duration for every single filter. Thus, I tend to think that per-wiki would be the best choice. But I'm also fine with having both options, if there's a good reason to.