Our last cruise on Adventure we had traditional dining in the MDR at 5. There was a sign at the entrance noting that the doors would close at 5:15. Our table was very close to the doors (yuck -that's a story in itself) and I can confirm that most nights, the doors were closed by 5:20 - 5:30. It was rare to see anyone show up after the doors were closed.
So to answer your question, your window would be 5:15 - 5:30 in my opinion. (more towards 5:30 so you don't have to stand in line waiting for the doors to open)
Hope that helps.