No latter than 4 pm the day of departure (first day of the boat trip). The boat leaves in the evening, but it takes a couple of hours to clear immigration at Padang airport and collect surfboards/luggage, get in the mini-bus and go to Padang harbour.  And usually there are a few things to do in Padang such as passport photocopies, exchange money, buy cigarettes, wax, a SIM card for you mobile phone, etc.