They generally take the reports, and depending on how many have been reported they will be put in a queue (SE only has so many people to investigate reports) After that they have to look at all relevant information available to them, as we don't know what they have we can't say, but the more information they have available the longer it will take.

For what is seen as low priority bots, they also don't ban instantly, people like mining bots, who aren't really inconveniencing masses, they will just do a blanket ban every month or so, it's the really annoying ones they ban instantly (like /tell bots).