Aiming for Messy?

I follow an Instagram account called TeachersThings that often has funny or inspiring posts for those who serve as educators. This morning in my Instafeed, I saw this one:

A screenshot from TeachersThings on Instragram.

My immediate reaction was, "YES!"

But the more I thought about this, I'm not so sure that is the right response.

Reasonable and Realistic Assignments

Hey there, teacher...

That assignment you gave your students today...how much time will it take your students to complete?

I suspect you have an estimate in mind for how long you expect it will take a "typical" student to complete it. But I want to slow you down there a minute... Who is this "typical" student? Does s/he really exist? How many of your real-life students are actually represented by this "typical" student?