How long did Isaiah go naked and barefoot as a sign against Egypt and Cush?

Answer: Three years

Isaiah walked naked and barefoot three years as a sign that Assyria would lead away the captives of Egypt and Cush.

“Then the LORD said, “Just as My servant Isaiah has gone naked and barefoot for three years as a sign and omen against Egypt and Cush,”— Isaiah 20:3 (BSB)