How much of London's story is devoted to describing the setting in to build a fire?

You should try to put your question under the correct title. Much of the story is concerned with setting. London was part of a movement called "naturalism". His natural wild settings defined his characters. Nature in fact was very much a character that encompassed or framed the narrative. It is this cold,wild and unforgiving setting that draws out the major thematic elements of the story.