Default for boostrap nav to show is 760px, if you want to change it, following is the setting you could do to customize the bootstrap. Following screen example show how to show or hide the nav icon at 1024px.
Just change the @screen-sm size. But if you want to customize bootstrap, most ealiest way I think is go to change at following site, http://bootstrap-live-customizer.com/ and get the variables.less and bootstrap.css.
Because if you just get the customized bootstrap.min.css, it will be hard to maintain what had been customized.