Based on his declaration in "Children of Time" - he said he has *always* loved her. In "Heart of Stone" he tells Kira (well, the Female Changeling in disguise) that he had gotten used to being alone, to being "nothing." He goes on to say, "Then I met you ...." Through their friendship he got to know her on an even deeper level, but I think for him the love was always there on some level from the first time he saw her.

I don't think he recognized it as 'love,' but there was certainly a spark there. The DS9 writers acknowledged that Odo was in love with Kira during the final scene from "Necessary Evil," which was early in season two. If he was already in love with her then, it must have germinated sometime earlier. If you look at their scenes together in season one, especially "Duet" and the episode with Thana Los (the name of the episode escapes me), Odo is very solicitous and 'soft' with Kira, a striking contrast to gruff, caustic way he treats everyone else.