Discipleship is a process – not a book, a class or a small group meeting. Books, classrooms and small groups can help the process, but discipleship is basically a progressive step by step journey toward the Cross.
Case in point: Nicodemus.
The first time we meet Nic, he is a fearful religious scholar who approached Jesus at night inquiring about signs and wonders. Jesus preached John 3:16 and more to him. Why at night? Probably because he did not want anyone to know he was interested in Jesus.
The next time Nic shows up in the Bible he is defending Jesus before the chief priests and Pharisees (John 7), thus risking his reputation and possibly his job. Is Nicodemus a born-again disciple at this point? Not sure, but he is definitely moving forward in the discipleship process.
Nic’s final appearance in the Bible is at the cross (John 19). He, along with Joseph of Arimathea, prepare their crucified Christ for burial. Now he is risking much more than his reputation; he is risking his life. If he was not a disciple before, he certainly is now.
I am not certain how many months or years passed from the first time Nicodemus showed interest in Jesus by asking academic questions, to the time he risked all by identifying with him at the cross. How long it takes does not really matter, as long as a person is in the process and the journey is heading toward the cross.