I have been a porn addict since I was a teenager and I so far I have not managed to get rid of it. However, I admit that it is sin and am willing to deal with it when God wants to help me. Right now I have not managed to find a girlfriend yet.
My question is: when would be the right time to confess to her about my addiction to pornography? Do I tell her before we get married, after, or hide it to the end?
"What shall we say then? Are we to continue in sin so that grace may increase? May it never be! How shall we who died to sin still live in it?" (Romans 6:1-2).
The problem is that you assume you will continue to sin and that is never the right answer. Nor is this a matter that is up to God to end. "No temptation has overtaken you except such as is common to man; but God is faithful, who will not allow you to be tempted beyond what you are able, but with the temptation will also make the way of escape, that you may be able to bear it" (I Corinthians 10:13). This means that God has already given you the opportunity to get out of this sin. It is up to you to take it.
It is probably because of your viewing of pornography that you are having trouble finding a girlfriend. Men who look at pornography tend to see women as sex objects instead of as real people. That doesn't lead to strong friendships.
Therefore, the only acceptable answer is to conquer this sin. Then if the subject ever comes up, you can tell her that you used to have a problem with pornography, but it was solved.