Well, I don't mean how long have you been born again, rather how matured are you? You can do a self assessment. HINT - Galatians 4:1-3 “Now I say, that the heir, as long as he is a child, differeth nothing from a servant, though he be lord of all;....even so we, when we were children, were in bondage under the elements of the world:” Others can tell your maturity by the way you talk and you can do a self assessment by observing whether are you still in bondage under the elements of the world. What do I mean? What happens to your body when the weather changes? What happens to your health when there is a common virus in the air? What happens to you when you eat spoilt food? What happens to your welfare when the economy hits the recession? But when the fullness of the time was come, God sent forth his Son,.....that we might receive the adoption of sons. Yayayaayaaah just like Paul “for I have learned, in whatsoever state I am, therewith to be content. I know both how to be abased, and I know how to abound: every where and in all things I am instructed both to abound and to suffer need. I can do all things through Christ which strengtheneth me.” Hail sons of God.
This is an interesting question of which I am currently searching the Scriptures in order to answer it for myself. So far I have identified what appears to be about 6 stages of spirituaL growth.
Child: has experienced the forgiveness [1 John 2v12 Amplified Bible Version]
Boy: knows (intellectual assent) about God, the Father, and has some awareness of the existance of the antichrist [1 John 2v13-14 Amplified Bible Version]
Please note that other versions of the Bible consider both the 'child' and 'boy' stages to be just 'child' stage.
Young Man: is victorious over the evil one, and has attained his strength and vigour from allowing the Word (Divine Revelation) of God to abide in their heart [1 John 2v13-14]
Son: is being led by the Holy Spirit [Romans 8v14]
Heir: sharing in His suffering first and then sharing in His glory [Romans 8v17]
Father: Knowing (having intimate knowledge and experience of) God in His infinite/ eternal nature [1 John 2v13-14]; knowing all things [1 John 2v20]; has reached Christ-level perfection and completeness [Ephesians 4v13].
I am still learning about this, and these are just my initial thoughts on the matter of spiritual growth. Wide open for inputs from other teachers of the Word, especially those who rightly divide and correctly analyse the Word according to the standards of teaching set in 2 Timothy 2v15.