Its hard to say as you didn't say what kind of poly the FD is giving you, length depends on the type of Poly. Not all poly's are the same .
It can be a "full scope" background, which goes back to your childhood and goes over everything from stealing money from moms purse, to drugs, to doing weird things with animals and your hatred of chili with beans...
Or it can be a "limited scope" (I have also heard some refer to it as an "employment trust") poly which only addresses certain things like theft, criminal acts, etc.
I took a limited scope poly that addressed national security and foreign contact for a specific task at work and it was only three questions, each was asked three different ways. With the hook up, prep questions, test, break down and scoring I was out in an hour.
What department is this that does poly graphs for the FD? Never heard of an FD using a poly for their hiring process before. Its expensive and unproven voodoo.