Sanctification in our lives occurs upon repentance and belief. Once a person repents and believes and is saved, God sets them apart from the unsaved. So sanctification occurs at salvation. However, it is also a life- long ongoing process from where we were not saved to the ultimate glorification. You should be able to look back over your life from when you were saved until now and see a firm pattern of growth in sanctification. You should also look forward and recognize that you are nowhere near where you want to be. There is always growth potential in our Christian lives. You are either growing in Christ, standing still, or backsliding. Or worse yet, never saved to begin with. You must consider this for yourself. Read the letter of 1 John. If you do not see progress in ongoing sanctification in your life, repent and believe. Ask Jesus to work in your life and speak to your heart through His word. Work through the tests in 1 John and see if you are truly saved.
1 That which was from the beginning, which we have heard, which we have seen with our eyes, which we have looked upon, and our hands have handled, concerning the Word of life— 2 the life was manifested, and we have seen, and bear witness, and declare to you that eternal life which was with the Father and was manifested to us— 3 that which we have seen and heard we declare to you, that you also may have fellowship with us; and truly our fellowship is with the Father and with His Son Jesus Christ. 4 And these things we write to you that your joy may be full.