In the novel Lord of the Flies how long were the boys on the island?

No time period is ever mentioned in the book. As there are no seasons in the tropics it is difficult to judge the passage of time. However the boys' hair had grown uncomfortably long, their clothes have fallen apart and the littluns have developed the distended stomach associated with malnutrition so some time has certainly elapsed. Nevertheless the naval officer still viewed them all as 'liitle boys,' so years certainly haven't passed. It is likely that the boys were on the island for several months but probably less than a year.