For how long was Brian missing?

Thirteen year old Brian Robeson boards a plane to visit his father, who is working in Canadian oil fields. He is traveling from the New York City area to the Canadian wilderness, and he isn't excited about the trip. He is upset with his father for divorcing his mother and upset about his father's secret, that he calls "the secret," that led to their eventual divorce. Brian is the sole passenger on this plane, accompanied by only a pilot. While in the air, the pilot begins to feel bursts of pain:

And now a jolt took him like a hammer blow, so forcefully that he seemed to crush back into the seat, and Brian reached for him, could not understand at first what it was, could not know. And then knew. Brian knew. The pilot's mouth went rigid, he swore and jerked a short series of slams into the seat, holding his shoulder now . . . 'Chest! Oh God, my chest is coming apart!' . . . The pilot was having a heart attack.

Before Brian or the pilot are able to communicate with anyone over the radio, the plane crashes; the pilot dies and the plane sinks to the bottom of a lake. Brian is left stranded in the Canadian wilderness with little but a hatchet to help him survive. 
In the epilogue, readers find out that Brian was alone in the woods for 54 days before he was rescued by another pilot. 

When the pilot rescued Brian he had been alone on the L-shaped Lake for fifty-four days. During that time he had lost seventeen percent of his body weight. He later gained back six percent, but had virtually no body fat—his body had consumed all extra weight and he would remain lean and wiry for several years.

The epilogue shares how many of the effects from living in the woods, such as his lower body weight and his deep thoughtfulness, would stay with him as he grew older. He learned, for instance, to value food deeply. The book explains that he would stand in the grocery store, spellbound by the abundance of food that people can access with ease. He remembers how difficult it was to search for berries and to hunt while abandoned, without any companion, for the two approximate months that he was lost. Through his great trials, Brian learns to appreciate the conveniences of the modern world that many people overlook.