Ideally, as you learn English pronunciation, you learn linking and blending at the same time. But, since it is not the case anymore, I would say that it depends on the capacity of the learner and the skill of the teacher to recognize that. In my class, I always inject linking and blending without my student knowing it because I don’t want him to feel that he is “learning” it, but instead I want him to acquire it “naturally” in class.
In addition, the mental and emotional readiness of the learner counts. As much as the teacher wants to teach linking and blending, but if the learner is not yet ready for it, it will be a waste of time and might lead to frustrations of both sides (the teacher and learner), and worse, it might de-motivate the learner.
Only the student can tell when the best time is and only a good teacher can find it out.