How long has God known you?
“Before I formed you in the womb I knew you; Before you were born I sanctified you; I ordained you a prophet to the nations.”
Since before He formed you in the womb. Thats what He said. Now let me see if I understand this. God "Formed" you and me in our mothers womb. He gave us hands feet, arms legs and formed every intricate part of us. But before he even started to form us, He already knew us. I'll get back to that.
And before we were born, while we were yet in our mothers womb. He set us apart (sanctified us). He made us special unique and with a special and unique purpose within His great plan. He created us with purpose. Jeremiah's purpose was to be a prophet to the nations. Gods purpose for each of us is unique. It's in His plan that we discover why we were created and find the deepest fulfillment. It may not be a calling to the nations but a calling to a neighborhood or to help a neighbor. Whatever your purpose in life, its what God created you for, don't miss it.
He knows you like nobody else because He created you, He created you before He formed you in the womb. This is the picture that I get. In heaven God creates living souls, there in His own hand He created you. Not the physical you that is temporary but the eternal you, created in His own image. He looked at you and you looked into His face and then just at the right time He sent you into your mothers womb and brought life to life, the eternal meets the temporal the soul meets the seed of the human body. I believe that you and I may have looked into the face of God and forever have a longing a hunger to look into His face once again. Our eternal soul will never be satisfied until we are with the father.
This is why salvation is so glorious. When our spiritual man, the eternal us, comes back into fellowship with the father and we see Him faith to face. We recognize Him, His eyes, His love. We don't literally see Him and our human eyes have never seen Him but we do see Him through the revelation of the Holy Spirit and just maybe our spiritual eyes have already looked upon Him.
1 Peter 1:7-9New King James Version (NKJV)
7 that the genuineness of your faith, being much more precious than gold that perishes, though it is tested by fire, may be found to praise, honor, and glory at the revelation of Jesus Christ, 8 whom having not seen[a] you love. Though now you do not see Him, yet believing, you rejoice with joy inexpressible and full of glory,
The "Image" of God is forever imprinted in us. We belong to God, we were created by Him and for Him. No matter where you find yourself in life you can always come back to your eternal purpose. You can always find your divine destiny, the reason why you exist is as simple as "you exist to please God".