The lifting up of a brazen serpent on a pole was a ceremony intended to show the Israelites that by looking to Christ they would be saved with eternal life, but by refusing to look to Him they would die spiritually. (See Num. 21:4-9; John 3:14-15; Alma 33:19-22.) At what time did the brazen serpent cease being a symbol of Christ and eternal life?
Here's answer to week's question
The brazen serpent was kept as a symbol in Israel until the time of Hezekiah, who broke it in pieces to keep apostate Israel of his day from burning incense to it. (2 Kings 18:4.)
Source: Bruce R. McConkie, Mormon Doctrine, p. 104, "Brazen Serpent."