It was released in December of 1998. Therefore, the 7th season was half over with. It was released in theatres while DS9 was on hiatus for a month. It was released between "Covenant" and "It's Only a Paper Moon." If First Contact is any indication, the producers waited 3 months in DS9 before the movie's events showed up. That would make it, at the latest, when the 8-part arc started at the end of the show.
The fact of the matter is that Insurrection should've been released in June of 1999 to save any confusion.