It's not something I see often as a healer, but several times when I queue in progress as a tank I see that less than 5 minutes of the dungeon timer has elapsed. I'll usually try to wait out a replacement as a healer, it's inconsiderate of other players' time to just drop out like that. Though I suppose I'd rather that than the player stays and harasses or griefs the party to get kicked, or plays lazily because they don't care.