Question for the pros

For those of you who have been in the business long enough to have taken on an apprentice, when did you feel like they were ready to tattoo? I ask because I have an apprentice that started a few months ago, and he's totally itching to tattoo. I just don't think he's ready. I apprenticed much longer before I felt I was ready to ink someone. What's your take? How long is long enough? And how do you tell someone that they're just not there yet? I don't want to discourage him but at the same time, I don't want him to think he's anything above an amateur right now.