You've been through this. We all have. Last night, we failed to finish Kel'thuzad. It was the first time we had to call the raid before clearing Naxxramas since we'd put it on farm status. A lot of it had to do with the absence of veteran raiders, substituted by some new guys who weren't quite on top of their game. But at a time when a lot of our veterans were bored with the content, we needed some warm bodies. Anyway, my point is, we called it after wiping at Kel'thuzad a few times and players were tired and wanted to go to bed.
Many times during our time playing this game, we've decided to call it. Usually in an instance, maybe a boss that's too hard to handle for our group at the time. In Battlegrounds, the equivalent would AFKing at some point where you know the game just isn't going anywhere (Warsong Gulch would be the likely culprit in those cases). I guess my question is when do you know when to stop? I've had experiences where 5-mans would have changes in their line-up just trying to down one boss -- when a player leaves in frustration, the others go to the public channels announcing "LF1M, last boss Heroic <insert dungeon here>".
But we have to stop sometime. When do you when that sometime is? After one wipe? Certainly not. Two? Three? I know we've wiped innumerable times on some progression bosses, only to end up calling it in order to continue another day. What is the straw that breaks the proverbial camel's back?