There needs to be a way to stop the timer when a dungeon group disbands after it's finished. Quite a few times now I have timed out before I can hand in the dungeon quest.
This is a real pain for players doing quests. There are a bunch of threads on this, but it won’t get changed any time soon. As a player there are some things you can try to do to help.
Make sure you tell the group you are questing in chat at the start. Then remind them before/after the final boss. This has always worked for me. Most players will be happy to wait. They don’t even have to stay in the dungeon, they just have to maintain the group.
Group with 1 friend so the group/dungeon can’t be closed before you both leave.
Find a guild that caters for players who are learning or questing in dungeons. This should be easier when the guild finder appears later this month. Lots of players want to do the dungeon quests and there are guilds out there that cover them.