How many years did God add to Hezekiah's life?

Answer: 15 years

God said He would add fifteen years to Hezekiah's life.

“I will add fifteen years to your life. And I will deliver you and this city from the hand of the king of Assyria. I will defend this city for My sake and for the sake of My servant David.’””— 2 Kings 20:6 (BSB)