Carousel speed in mobile

Folks, What is the optimum speed for carousels in a mobile website? For you to answer better -

  • optimized copy (say 6-10 words for mobile) as against 12-20 words in desktop
  • Maximum of 4 banners in carousel
  • When i saw few websites, realized they set speed between 5-8 seconds
  • Some websites have carousels but they don't auto rotate in mobile version