I guess that I am not talking so much about our eternal destiny than our destiny with respect to our part in His kingdom on earth. This deviates from your orginal concern. But it only took one stroke of Moses' staff to break his path to his ultimate destiny. It only took Esau one meal to separate himself from his destiny.
As for 'can we walk out of our eternal salvation?', I believe it is difficult but possible. I believe the scripture is too clear in its warnings for it not to be possible. Since the scripture does not define quantitatively how far we can go before we accomplish that feat, I cannot, nor can anyone else answer your question definitively.
While I cannot come up with chapter and verse, there is a passage in Psalms, I believe, that says He looks past our sins, but judges our unrighteous attitude or heart. I will look diligently for the reference because I believe it to be pertinent to this discussion.
I believe it is more a matter of attitude than a matter of a particular quantity of sin. Are we pursuing Him? He is careful to guide our steps on the paths of righteousness for His namesake. Are we pursuing our own self interest and denying His call on our lives? We are walking on dangerous ground.
Romans 1:21-32
For even though they knew God, they did not honor Him as God, or give thanks; but they became futile in their speculations, and their foolish heart was darkened. [22] Professing to be wise, they became fools, [23] and exchanged the glory of the incorruptible God for an image in the form of corruptible man and of birds and four-footed animals and crawling creatures.
[24] Therefore God gave them over in the lusts of their hearts to impurity, that their bodies might be dishonored among them. [25] For they exchanged the truth of God for a lie, and worshiped and served the creature rather than the Creator, who is blessed forever. Amen.
[26] For this reason God gave them over to degrading passions; for their women exchanged the natural function for that which is unnatural, [27] and in the same way also the men abandoned the natural function of the woman and burned in their desire toward one another, men with men committing indecent acts and receiving in their own persons the due penalty of their error.
[28] And just as they did not see fit to acknowledge God any longer, God gave them over to a depraved mind, to do those things which are not proper, [29] being filled with all unrighteousness, wickedness, greed, evil; full of envy, murder, strife, deceit, malice; they are gossips, [30] slanderers, haters of God, insolent, arrogant, boastful, inventors of evil, disobedient to parents, [31] without understanding, untrustworthy, unloving, unmerciful; [32] and, although they know the ordinance of God, that those who practice such things are worthy of death, they not only do the same, but also give hearty approval to those who practice them.
Ultimately, the creature that we always tend to place above God or in place of God is none other than ourselves. That rebellion, if we insist upon it will eventually lead to God giving us over to our own lusts.
The Father is the one who can answer this question, not anyone here. And I believe that it is different for every one of us. And may differ during the course of our lives. Ask Him.
Blessings,
Doug