The other day I was in a pantheon game where a player on my team was spiking to 2k.  I conceded and asked him to do the same, as the game seemed like it would be unplayable.

However, he didn't respond and the game went on well past the point where we could concede without taking a loss, which is the first problem. Once we were past the three minute mark he could have conceded for a completely different reason than I conceded and forced a loss on me.

His connection problems subsided and we played a good game up until the very end when they returned.  After 20 minutes of gameplay I had to watch him begin to time out, which would have auto-conceded, and auto-lost the game for us since I had conceded at the start. I lucked out and his connection held on long enough for us to get the win, but the connection screen popped up and counted up to 15 at one point.

So long story short I think that once you concede it should un-concede (new word!) 30 seconds to a minute later.