Bootstrap is based on a 12 column grid, and has four basic breakpoints:
xs (smallest screen) 480px
sm (small screen) 768px
md (medium screen) 992px
lg (large screen) 1200px
For page content, we keep the width of the content area at a comfortable reading width that works out to be: col-xs-12 col-md-8. For examples, please refer to the Page Patterns.