Yep. Don't leave until your done if your doing Hard Mode. Normal mode should let you come back and pick up again, but you lose the achievements you would have earned.
If you do leave, you got about 15 minutes before you lose the Instance, then it may reset no matter what. I seem to remember them doing something with this Trial so that people could come back (for training purposes mostly) so that might keep your instance as long as the quest is kept.
Edited by Nestor on July 15, 2016 7:24PM
Enjoy the game, life is what you really want to be worried about.
PakKat "Everything was going well, until I died"
Gary Gravestink "I am glad you died, I needed the help"
You can leave and come back and stay in the same round and stage but lose leader boards and score.
If you leave and log off then come back, then you'd be back to round one of that stage again with no score.