Let's make sure we're talking about the same passage. Is this what you're talking about?
1 Peter 3:18-20 KJV
(18) For Christ also hath once suffered for sins, the just for the unjust, that he might bring us to God,
being put to death in the flesh, but quickened by the Spirit:
(19)
By which also he went and preached unto the spirits in prison;
(20) Which sometime were disobedient, when once the longsuffering of God waited in the days of Noah, while the ark was a preparing, wherein few, that is, eight souls were saved by water.
Diagram that passage for a moment. "By which" points to which phrase?
Jesus didn't preach by being put to death in the flesh, he preached by being quickened by the Spirit. That quickening by the Spirit
is an event that declared victory precisely three days and three nights after he was put to death in the flesh. Paul explains this so as to where we can avoid any confusion:
1 Corinthians 15:44-45 KJV
(44) It is sown a natural body;
it is raised a spiritual body. There is a natural body, and there is a spiritual body.
(45) And so it is written, The first man Adam was made a living soul;
the last Adam was made a quickening spirit.
I think you may be mistaking the word "preached" to mean "stand in front of a crowd and talk persuasively." That is one application of the word, but it essentially means "proclaimed." As such to your question of "when" the answer would be "when he was risen, having been quickened by the Spirit." Had Christ merely perished on that cross there would be no victory to preach. Yet his resurrection his victory over those spirits is proclaimed.