Relation between transit time and rise time

In the homework I have been working on, I encountered with such a statement:

If the rise time over transmit time in a circuit element is greater than 6, the circuit element can be treated as a lumped circuit element.

However, I couldn't understand where 6 comes from. If somebody can explain this, I will be really happy.

Thanks.

• It's just a rule of thumb, similarly to how after 5 time constants you can assume the signal settled. It is a decent proportion where the lumped approximation becomes "good enough" – jbord39 Feb 7 '17 at 17:31