How long does it take for an average born-again Christian to get over their passion about Jesus?

The fact of the matter: If you are truly saved by God The Lord Jesus Christ our Lord and Savior, you will never stop preaching God The Lord Jesus Christ our Lord and Savior holy word the bible ever. You will not be able to get enough of God The Lord Jesus Christ our Lord and Savior holy word the bible and you will want more and more each an every day.

ALL Christians are born again in baptism.  And I've never met ANY Christian who talks non-stop about Jesus.

Not long at all . To keep yourself actively passionent for Jesus you have to keep obeying all of his sayings .

"Get over"?  "But GROW in the grace and knowledge of our Lord and Savior Jesus Christ."  2 Pet. 3:18

They were not passionate for Jesus, but for the power and ego trip such pretense provides.  Jesus is their excuse, not their inspiration.  They claim Jesus said not to lie, but they ignore that completely.  With intent.

My excitement for the Lord is never ending!  :D  Here is the truth:  Death leads to immediate heaven or hell. The only way to have eternal life, the only way to be in heaven and avoid hell, is by believing in the Lord Jesus Christ (John 6:47).  It is that simple and that easy.  It is too late to be saved after death.  Jesus is God and Jesus died on the cross and shed His blood to pay for all of our sins in full, and then Jesus was buried, and then Jesus resurrected from the dead (1 Corinthians 15:3-4).  The Lord Jesus Christ paid for our sins with His death and blood.  Nothing else pays for our sins, not works from us or anything else.  You cannot add any works to getting into heaven (Romans 4:5).

As long as it takes not to aggravate you or win the lottery.

They don't "get over" it... They just know that there is a living God and want to share this experience and truth with others.

It all depends 

Luke 8

11Now this is the meaning of the parable: The seed is the word of God. 12The seedsc along the path are those who hear, but the devil comes and takes away the word from their hearts, so that they may not believe and be saved.

13The seeds on rocky ground are those who hear the word and receive it with joy, but they have no root. They believe for a season, but in the time of testing, they fall away.

14The seeds that fell among the thorns are those who hear, but as they go on their way, they are choked by the worries, riches, and pleasures of this life, and their fruit does not mature.

15But the seeds on good soil are those with a noble and good heart, who hear the word, cling to it, and by persevering produce a crop.

It's up to us to keep our lamps burning. 5 of the 10. Virgins waiting for jesus ran out of oil and when He came they were not ready. 

Luke 12:35

Be dressed for service and keep your lamps burning

NEVER IF THEY ARE TRUELY BEGOTTEN OF YHVH GOD IN YESHUA THE MESSIAH.