If your first time on the trail, I would err on the side of caution and make every effort to be no higher than Mirror Lake by sunset. There are some very tricky turns above Mirror that have been missed in the dark with bad results. I would also not assume that you will be moving a lot faster downhill than up, until you descend to Mirror, so you may just want to cut your time from start to dark in half, and set your turn around time for say noon. You also do NOT want to be on the chute after it has iced up again in the afternoon, so an earlier turn around than you might expect would be called for for that reason as well. Others can give you better specific turnaround advice, but these are the major considerations that I would be aware of.
