When will you have the assurance that you have eternal life in Christ, as an absolute certainty?
What do you mean by "absolute certainty"? If that means a mental state in which faith and hope are no longer necessary, I doubt that day will come in this life. As Paul wrote,
16 The Spirit itself beareth witness with our spirit, that we are the children of God:17 And if children, then heirs; heirs of God, and joint-heirs with Christ; if so be that we suffer with him, that we may be also glorified together.
18 For I reckon that the sufferings of this present time are not worthy to be compared with the glory which shall be revealed in us. (Romans 8:16-18)
The Spirit bears witness that I am a joint-heir with Christ. If I continue to suffer with Him, I will be glorified. I am as certain of that as I can be of anything.
However, I do not yet see the glory that I am to inherit. Until I do, I must wait with hope and patience:
24 For we are saved by hope: but hope that is seen is not hope: for what a man seeth, why doth he yet hope for?25 But if we hope for that we see not, then do we with patience wait for it. (Romans 8:24, 25)