I would leave at 7:30am. This sounds early but Sometimes the bus stops at the Boardwalk and the Swan Dolphin. You should reach the park around 8:30 - 8:40 if the bus does make these stops. Also, getting to the park early is necessary if you want to experience rope drop. disney will begin letting guests in around 8:30am. Thus being early gets you into the park early and hopefully at the front of the line for rope drop.