A well-worded and thoughtful blog post takes time and effort. It also interrupts other development work, which the context switching makes doubly costly. So I don't think it's a "15 minutes over lunch" affair exactly.
It's also a good time to take stock of where you are in a project, and where you are heading. making yourself put your progress into words forces you to think about the overall view. We started an internal blog/status report page for our projects, and simply writing a post - even if it's basically to myself - is helpful.