"Oops GTG TTYL bye" – heard that one before, in the middle of an instance? You don't want to be that guy.
Today's World of Warcraft is a far cry from Tomb Raider or Lego Indiana Jones, which can leave you writhing in your chair for what seems like an eternity before your next save point. For an MMO, WoW's gameplay has become reasonably bite-sized – so don't get caught virtually chewing with your mouth open by dropping out of groups early.
True, many instances can be completed so quickly and many players are so experienced that dashing through a quick instance can seem almost trivial. But a group is a social creature, and accepting a group invitation means accepting a reasonable responsibility to treat the other players' time with respect.
Don't accept a group if you know you can't stay. If it's getting close to dinnertime or class or work or anything else that'll bring your game time to a screeching halt, don't foist that problem off onto your groupmates. At the very least, speak up before the group begins. Tell them long you expect to be able to stay, and let them make the call on whether to move forward with you in the group or look for someone else who can go the distance.
If something unexpected comes up, offer to look for a replacement. Check with guildmates and friends to see if anyone you know would be willing to plug the hole you're leaving.
And if you've run into a group you can't abide playing with one second longer (whether because of a personality clash or because the group simply isn't up to the task at hand), take time to make a polite excuse before bowing out. Yeah, we've been there, too -- just grit your teeth and do it. Don't leave your group hanging in the lurch, wondering what happened and whether or not you're coming back. Be clear, be polite and don't burn bridges that could lead to future groups, guilds and friendships.