In theory, this can start at any time. Remember, learners are encouraged to gain as much practice as possible on as many road types as possible. Do keep it reasonable, though. Do not encourage a fast track approach. The pupil should agree with the their instructor a methodological learning plan that will usually take up to several months. So try and avoid taking your teenager on a national speed limit dual-carriageway or a very busy, complex roundabout if they have not come to these topics with the instructor.There is the safety for other road users involved as well.
In the early weeks, anything to do with driving is new to a novice driver. And like a sponge they should be soaking it all up. Do not worry about putting across bad habits or a lack of driving knowledge at this stage as it is purely the basics they are learning.
When the learner is reaching their driving test stage then a lack of driving knowledge can interfere. Such as checking blind spots before moving off from a parked position. This is critical to safety and has been taught for many years, but how many experienced drivers have forgotten to do this key step.
A common mistake by parents, friends or relatives and even instructors is to tell the pupil exactly what to do rather than coach them through it. Drivers have to think for themselves in the end.
Taking the learner out for more practice should be encourage at all times. Once a week with a simple drive is fine.