The answer to when depends on what 'rich' means. As has been noted by only ~1 answer so far, this has nothing to do with material or worldly wealth.
Jesus is 'heavenly', he is the fullest expression of God on earth - ever. Heavenly is not a place as such (God is spirit), but a state of holiness, of truth, grace, love, etc. to their greatest extent - all found in God alone - to the point that John says, 'God IS love'.
Therefore, Jesus' wealth/riches are spiritual in nature and barely apprehended by corrupted humanity. Jesus taught specifically that being rich in worldly terms was contrary to entering the Kingdom - he always led by example so we must look elsewhere to worldly riches in this passage of 2 Cor 8:9
I tell you, it is easier for a camel to go through the eye of a needle than for someone who is rich to enter the kingdom of God. Matt 19:24
To be 'worldly rich' is to have excess - to have excess is to be selfish. Was Jesus selfish? The analogy to us is also true -
For you know the grace of our Lord Jesus Christ, that though he was rich, yet for your sake he became poor, so that you through his poverty might become rich. 2 Cor 8:9
When we 'become rich', it will not be for worldly things!
The process of 'becoming poor' does not happen by fiat at some particular point in his life. Jesus had his own will - which often differed from the Father. He was empowered, by God's spirit in him, to submit this self-will to God in humble obedience. Obedience we are told came through suffering.
(Jesus) in the days of His flesh, having offered up both prayers and supplications with loud crying and tears to the One being able to save Him from death, and having been heard because of reverent submission, though being a Son, He learned obedience from the things He suffered, and having been perfected, He became the author of eternal salvation to all those obeying Him Heb 5:7-9
The challenge for Jesus was that the tests and the suffering increased until the garden through to the subsequent walk to Golgotha. At ever increasing need for the Father's provision, he became poorer until that final triumphant breath - "It is finished!"
Only through Jesus' perfect submission could we inherit eternal life, as he subsequently did. To achieve the great victory over pride and self, evil and death, Jesus became poorer - finally being completely empty of self.
Satan was not defeated by Jesus alone - but the power of God's love in Jesus who always looked to the Father for all his needs - by the choice he had as a human.
God didn't defeat Satan alone - but by the willing submission of His son to rely on Him for ALL things.
I can do nothing by Myself; I judge only as I hear. And My judgment is just, because I do not seek My own will, but the will of Him who sent Me. John 5:30
For I have come down from heaven, not to do My own will, but to do the will of Him who sent Me. John 6:38
Only by Jesus total spiritual poverty - through God's abundant grace, could there be a victory at all - we now share in that victory and will enjoy the riches with God and His son forever more.
In summary, Jesus becoming poor took place from at least his baptism and his receipt of the HS until his last breath.
Jesus could sin - his prayers confirm that (Heb 5:7) If he did sin, it would be because of pride. His tempter gave pride a 'run' and was found wanting - God gave this majestic, beautiful Archangel the flick. The tempter tried the same ploy on Jesus - to fool him into thinking he could go ahead without God - the kingdoms of the world would be his (allegedly) that Jesus could rule the world and that he wouldn't need God anymore - sound familiar? (second Adam)
It's not that Jesus had pride, but he had the potential for pride to get a start if he put his will before God's. Just once and he was doomed, we were all doomed. To misunderstand the logos as God, is to misunderstand what God was doing through His 'logos become flesh'. God's word now had it's own will in Jesus! This is lunacy from a human standpoint! But God is God, His ways are so much grander than our best moments. God's word, which always returned to Him having accomplished what He sent it to do - was now a human with his own will!!! (Is 55:11)
While Jesus did indeed have his own will, his own desires and ideas, his challenge above all was to put God’s will before his own. This wasn’t automatic, it was a test, a trial and a trust in God over everything other human input he received.
Jesus had no illusions about going it alone - not for long anyway. Because he and the Father were one in purpose, uncomplicated by the deceptions of sin and self, in spite of the temptations, he was able to keep his eye on God and the goal they were seeking together by God's spirit filling him.
God defeated evil through a man. A man defeated evil with his God. Jesus is the forerunner for us (Heb 6:20) - we too are men and we have the same God as Jesus did.