I can only assume that TV is correct about the big difference between fixed wing and heli - my total heli time doesn't qualify me to comment.
But, from a fixed-wing position, my answer would be asap. I would generally start with the first lesson after the trial lesson, ie once the student has decided to take lessons regularly. I'd only have students doing radio work on the ground - in the air, I'll take over and let them concentrate on the flying.
To start with, I'd tell them exactly what to say, and have them presss the ptt button and say it. I'd handle the readback - all I'm really interested in right now is getting over any shyness of using the radio, I don't really care about exactly what's being said. Once the student has got over the initial shyness, I would continue to handle the readbacks myself, but point out to the student how the readbacks work. Generally within a few hours, they'd be ready to do the readbacks themselves - although the readback of the initial call, which includes several pieces of information (taxy route, holding point, runway, QNH) I'd usually expect to have to help with, by either writing down the instructions for the student to read back, or helping the student anticipate the reply, depending on the circumstances and the student.
Radio in the air I would only introduce once the student is in the circuit. Get them doing the downwind call and the final call - not on their first circuit session, because they'll be too busy flying, but very early on in circuits.
That's all the radio they need to be able to go solo.
After solo, I would do a dual trip on leaving and re-joining the circuit, followed by a solo repeat of that trip. The briefing for this trip should include use of the radio outside the circuit. More radio, including freecalls and "pass your message", I would introduce during the navigation phase of the course.
Of course, the key is flexibility. I had one student for whom English was not his first language, and how to get him comfortable on the radio required a lot of thought and effort from myself and my colleagues. Also, the environment where you train (is your airfield ATC, FISO or A/G, do you have a nearby LARS facility, do you do your circuit training at your home airfield, etc, etc) will affect how to introduce radio to your students.
Hope that's given you a few thoughts to ponder on.
FFF
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