How much ? We can't know. Only God knows. We don't necessarily have to know.
Our christian faith teaches us that :
- there IS free will and personal responsibility (or else we are not free creatures but mere puppets)
- none of us is fully "free" since our decisions are affected by many factors : health, education, culture, past experiences, other creatures etc.
- we are responsible for ourself and each other as far as we can. God respects our dignity and doesn't "assist" us like babies. It does however take care of all of us to ensure our personal growth through what we call "Providence". But we have to freely respond.
Recent social studies show that we are all influencing each other more than we may have previously thought. For example, divorce and suicide have been shown to be "contagious", i.e. a person doing it greatly increases his relatives chance to do the same.
It seems also that God leaves to us the task of propagating knowledge of Him (at last up to a certain mesure) though He could reveal himself to all people if He wanted to.
So we are definitely at last partially responsible for each other salvation, at last the part that doesn't depend on their own free will.
But evaluating "how much" we affect each other would require a full understanding of human mind and social interactions. We are VERY far from such knowledge.
Saints testimones (ex. st Joan of Arc) show that they honestly and genuinely didn't dare to presume that they were "in God's grace" which can be understood as "contributing to their own salvation and other's salvation". So saints didn't know either.
On the other hand, since Christian faith is about our personal, loving relation to God, such "how much" questions can be seen as irrelevant. Evaluating "how much" may imply the desire to "do only one's share" or "evaluating one's contribution", so introducing some kind of "due" or "contract". This is not how a love relation is supposed to work.
So a way of answering for a christian may be "I don't now, I can't know, I don't need to know. I just want to do the best for my brother humans out of love for God and them, without counting. I'm definitely not doing all I could but I'll do my best to sincerely and creatively help everyone's growth and have confidence in God's help and forgiveness on that matter." This is how saints like St Theresa or St Vincent of Paul expressed it.