How many days did it take to complete Jerusalem's wall?

Answer: 52

The wall was completed in fifty-two days, on the twenty-fifth of Elul, and the surrounding nations realized God had done it.

“So the wall was completed in fifty-two days, on the twenty-fifth of Elul. When all our enemies heard about this, all the surrounding nations were afraid and disheartened, for they realized that this task had been accomplished by our God.”— Nehemiah 6:15-16 (BSB)