The day the servers shut down. Sure, may take a few days break here and there. But for me, ME3 is the ONLY multiplayer I play. Yes, thats means I don't play Borderlands 2, Call of Duty or Modern Warfare. Never have, never will. Despite my lack of interest in multiplayer I'm not at all surprised with my love for ME3's.
I remember fondly when I had finally finished insanity on ME2 and completed all the trophies. I had played paragon and renegade (as much as I could stand to be a renegade, at least) and even tried a neutral approach. As much as I deeply loved the game, after so many playthroughs I finally was losing interest in playing. Not interest in the series or the game, by any means. Just didn't feel as excited to replay the same missions over and over after having already 'perfected' it.
I used to think "man, I sure wish this game had some co-op or multiplayer. I would love to keep playing in the ME universe." Couple years later, here we are.
I remember when it was first even rumored that ME3 would have muliplayer. I was excited, very excited and hopeful that it was true. Pretty much everyone else, or at least those who were most vocal, were oppose to the idea. When it was confirmed, I was quite upbeat about it. I knew it would be something I'd want to play, despite the swarms of people who seemed to cry about how horrible it would be, despite the game not even being out yet. I still don't know why everyone was so concerned about it. Even defended it on a few occasions.
I had faith that BioWare could and would make a great multiplayer experience and a great sequel. I was right about one of those. Still recovering from the melancholy of the other. Least I still have the multiplayer to fall back on.