Ask your photographer how much time s/he likes between the end of the ceremony and sunset for taking pictures (this will depend on size of wedding party, size of families, and number of guests that need to be photographed).
Then ask your officiant how long the ceremony takes (including special readings, family unity celebrations, etc.).
Work backwards from there to find a start time. Build in a little extra time (maybe 15 minutes) as a cushion.
Best of luck,