According to Jesus, in the Father's eyes Christ and his Church are one. Paul illustrates this with the analogy of a human body. He says Christ is the head, and we are his body – bone of his bone, flesh of his flesh. "[God] put all things under His feet, and gave Him to be head over all things to the church, which is His body, the fullness of Him who fills all in all" (Ephesians 1:22-23). "We are members of His body, of His flesh, and of His bones" (Ephesians 5:30).
Do you see the implication here? When the Father loved Jesus in eternity, he loved us, too. Indeed, when man was but a thought in God's eternal mind, the Lord was already numbering our parts and planning our redemption.
"Just as He chose us in Him before the foundation of the world, that we should be holy and without blame before Him in love" (Ephesians 1:4).
God has always loved both his Son and us. His love is as everlasting as he is: "I have loved you with an everlasting love" (Jeremiah 31:3). "Our God and Father, who has loved us and given us everlasting consolation" (2 Thessalonians 2:16).
Nobody can gain God's love by any godly thing he or she does. Jesus didn't earn his Father's love by going to the cross, or by his obedience, or by loving his Father. Likewise, God didn't start loving you the day you repented and received Christ. He didn't suddenly love you once you began to obey his Word and walk in the Spirit. You were already loved from eternity.
How long has God loved you? He has loved you since he has existed, because God is love. It is his very nature. He loved you as a sinner. He loved you in the womb. He loved you before the world began. There was no beginning to God's love for you, and there is no end to it.