In the previous chapter Sun Tzu counted a thousand gold a day. Once you know the price, the next question follows by itself. How long can we hold out.
His answer is unexpectedly blunt. He does not tell you how to endure. He tells you not to go long.
兵貴勝, 不貴久
bing gui sheng, bu gui jiu
War prizes victory. It does not prize duration. (The Art of War, Waging War, translated directly)