You can take however long the festival is going on to finish it. They stamp your book with a stamp that is not dated so they have no clue when you started it and they don't care. In the past, at all the festivals that have some sort of stroll like that, I've usually take 5 to 6 days to complete (the length of my trip). The cookie stroll is particularly difficult for me since I'm usually solo and more than two cookies a day is impossible for me to eat.