It depends on what you're drilling. If the language item is the target language for the lesson, for example a grammatical structure, then drilling is usually done after this has been elicited or presented. In a PPP lesson, for example, it is the very first practice stage – drilling in effect gives your learners extremely controlled practice of using the target language. If the language you're drilling is vocabulary, then you might do a quick drill as the word or expression comes up (for example while pre-teaching vocabulary for a reading activity or during a lead-in task) and then repeat it at the end of the lesson during a recap of new vocabulary.