Sometimes, it’s no research time at all, sometimes it’s months.
If I am writing a personal Gotham story with characters I created or know like the back of my hand, sometimes only minimal research is needed—I shouldn’t say NO research, because there’s always something I need to look up.
But if it takes place in another country or there’s lots of history, it can be a year or more to really try to know the subject before tackling the issue.