I have decided to put my oar in here. For a start, there is quite a process to go through to obtain salvation and get the assurance of it. Firstly, you have to see that you are a sinner, a complete sinner, a wretched sinner worthy only of condemnation; and rightly so. Before we can get past first base, we have to have a very clear appreciation of the fact that we are sinners who justly deserve eternal hell. Too many people walk up an accept Christ without going through that process, and it is no surprise that they last only a short while before they either fall back into their sinful life or continue a false Christian life full of pride and self righteousness.
The Gospel only becomes good news to those who are fully convinced of their wretched sinful state and also convinced that in no way will they ever be able to live a sinless live, acceptable to God.
Then, the next step is to stand before the cross of Christ and fully appreciate what He did for us there. He died to make us fully justified and acceptable to God. We need to accept Jesus into our lives on that basis.
Then we need to seek the Lord for the assurance that we are fully saved, and that our step of faith in believing that Jesus died for us on the cross, and our acceptance of Him into our lives has justified us before God. We need the Holy Spirit to put that witness into our hearts. It might not come automatically. According to many examples of godly men and women through the ages, it seems that it has to be sought for over a length of time and with all your heart.
It took John Bunyan eight years of struggling before he got that assurance from the Holy Spirit. That is a big difference between the easy believerism where someone can waltz up to the altar and mumble some prayer of committment and then believe that they are truly saved.
The trouble with many of today's Christians is that they have done just that. Gone up to the altar, probably under pressure from a group of enthusiastic people who want God to work more quickly in the person than might be appropriate, and then come away not knowing exactly what he did, other than 'getting religion' and then spending the rest of his or her life struggling along trying to be 'holy', and probably ending up in bondage to self righteousness and filthy, stinking pride, and then falling down into Hell when he or she dies - merely because they did not make sure that their salvation was sure.
The world is full of backslidden 'Christians' who have gone this way. They never waited for God to work in them to give them the assurance of their salvation, so they fell away after a time of trying because they knew that did not have what it took to live the Christian life.
It is significant that those who seek long and hard with God for the assurance of salvation, and then get it through blood, sweat and tears before God, do not usually fall away from Him afterwards, but go on a live very strong Christian lives.
So, how much sin does it take to lose your salvation. Well, if you never had salvation or the assurance of it in the first place, then it just takes the effort to walk out of the church and not go back.
But for those who have done the hard yards with God and received the assurance of salvation from the Holy Spirit, no amount of sin will cause them to lose their salvation, because they will love God so much that they will not leave any sin unconfessed and unforgiven.