A timing belt is typically made out of rubber, though some are manufactured out of polyurethane or neoprene. Meanwhile, its inner structure is then reinforced with fibers, and you'll even find robust fabric covers on the toothed surface of the belt. Given the fact that a timing belt is mostly made out of reinforced rubber, the excessive heat from an engine will, over time, cause that rubber to harden and crack. In other words, a timing belt is not made to last forever. In general, a timing belt is usually good for between 40,000 to 100,000 miles before it needs a replacement. Meanwhile, a timing chain (often made of metal) lasts longer, often between 80,000 to 120,000 miles.