As is often the case with love for me, I'm not sure there was an exact moment when I "knew". I can say with certainty that I did not love him in 101 and I did in 122. And working backwards from the end of Season 1, I can point to specific examples when I felt love for him. An exact moment, a single scene or episode where the line is crossed however, just isn't possible for me to identify.
I do think that with hindsight I see Brian as more loveable earlier on than perhaps I did during my first viewing of the series. But that's because I now know how he'll be and what he'll do, whereas initially I did not. For example, now even as early as the ending of 102, I can feel my love for Brian lapping at the corners of my rather hardened heart. Though I'm sure that I didn't feel that way about him the first time I saw it. But love does have a way of sneaking up on you when you're least expecting it!