I was just at the DTW LH lounge last week. It closes at 6:30pm, following the departure of the LH flight.

In my experiences, the lounge opens as early as 9am. I do not know of it being open any earlier.

As other posters have noted, be sure to have your *G card. If you do not have it, you will be rejected. Having 1K or Prem Ex on your boarding pass will NOT suffice.

The Senetor lounge is specifically what I am referring to. The business lounge opens at 2:30 and closes with the Senetor lounge at 6:30.