To me the very question is flawed and nonsensical, since:
1) You're talking about some arbitrary default size, and not the needs of the content.
2) You're talking in pixels, which is the LAST measurement that should be used in layout.
Content dictates your semantic markup -- content + semantic markup dictates layout. As such your initial layout should adapt to the needs of the content... which is why I build for desktop FIRST according to the needs of the content, typically as wide as the widest I think is practical -- which is usually 20em per sidebar with no column allowed any wider than 48em, as long lines of text are shite wider than that.
Then you keep shrinking the window and adding media queries -- in EM -- as needed.
I never think in a default "size", I let the content and needs of the content (and user) dictate that.
It's like the old BS of "design for 800 friendly at 768px" or "1024 friendly at 976px" BULLCOOKIES, that resulted in broken inaccessible garbage sites.
Fonts in EM, padding in EM, max-widths and fixed columns in EM, media queries in EM, with the CONTENT and content-limits determining how many EM to use.
It also doesn't help you say 'breakpoint' -- SINGULAR. Typically I have anywhere from three to six. Again, make it big, then shrink the window. When the layout breaks, fix whatever broke at that width (in em) with a media query, then lather-rinse-repeat layering queries until your layout works down to around 12 to 16em. (192 to 256px for 16px users, 256 to 384 for users like myself)
Your breakpointS should be based on the needs of the content, NOT some arbitrary predetermined number!