"Jesus had what many scholars call his 'Retreat Ministry,' a period of time that was dedicated completely to the disciples, when he retreated to places the crowds would never follow, when the disciples could be immersed in relationship and have complete access to Him. Here's the interesting thing: Most people think that at least eighteen months of Jesus' public ministry was this time.
That means at least half of his time was spent with these twelve guys.
He believed so powerfully in discipleship that he basically put all of his eggs into that basket. (It's worth nothing that it paid off.)"
Mike Breen from Building a Discipling Culture