Gap between H4 biometrics and final H1B extension

Hi - I am awaiting my H1B extension (filed Sep 29, 2020). I recently received notice for my dependents’ (H4) biometrics to be done in late February 2021. A couple of questions:

  • What is usually the gap between H4 biometrics appointment and H1B extension?
  • I was considering applying for premium processing when the biometrics appointment came in. Is it worth doing that now, and does the biometrics appointment signal my case is now close to final renewal?
    Will be grateful for any insights and experiences.