I watched DS9 from the very first episode and liked it, so I can't really say when it hooked me. I mean, for me, a show has to hook me from an overall premise perspective even before it starts.
A better question in my case it to ask "When did it dawn on you that DS9 was something EXTRA special" - not just another good Trek show, but instead had the potential to be a GREAT (indeed, the greatest) Trek show.
And for me, that revelation dawned over the course of two weeks - the two weeks that Duet, and then In the Hands of the Prophets aired.
By the end of those two episodes, I knew that DS9 was something EXTRA special.