If it's an EMH, 7:30. On our last trip we did the keys to the kingdom tour and we had to take the boat to the Contemoray, then walk the seven minute walk over. The monorail takes more than double this time.
Jonimce is correct. If your reservation is for a non EMH morning, take the blue flagged launch, which starts operating every day at 7am to the Contemporary and walk from there.