It’s typically recommended to start using a Qbank once you’ve covered a good portion of the basic sciences in your coursework. This ensures you have sufficient background knowledge to make effective use of the Qbank.  However, do not wait until your final leg of USMLE Step 1 preparation. Getting used to applying what you have learned in USMLE-style questions early on will ensure you have mastered the approach to the questions by the time you take the exam.