Many people know exactly when they responded to God's call and gave their lives to Christ. In my own experience, I can still vividly remember the night I went forward at an evangelistic meeting and committed my life to Jesus.
But this isn't always true. My wife, Ruth, for example, grew up in a fine Christian home; her parents were medical missionaries in China. From early childhood she heard the Bible and learned about Jesus' love—and as a result she can never remember a time when she didn't believe in Jesus and have faith in Him. Yet Ruth to this day is the strongest Christian I have ever known.
You see somewhat the same pattern in the Bible. Paul had a very decisive conversion experience; on the road to Damascus, Jesus appeared to him and from that moment on he dedicated his life to Christ's service. But Peter, on the other hand, wavered in his faith, and the Bible doesn't tell us exactly when he "crossed the line" and decisively committed his life to Jesus.
The most important question for you (or anyone else) is this: Are you now trusting Jesus Christ alone for your salvation? Do you know that if you were to die tonight, you would awaken in Christ's presence? The Bible is clear: "He who has the Son has life; he who does not have the Son of God does not have life" (1 John 5:12). If you are trusting Christ, your salvation is secure.