Here's what I was taught- the independent variable is the guerrila's capacity to conduct violence.

So,

An insurgency can be labeled a civil war once the guerrila builds the same capacity to conduct violence (military mass) that the host nation possesses.

Applied loosely, this does not have to mean the the guerrila possesses the same amount of tanks as the host nation. That's why one could justify Iraq moved into a civil war between late 2005 and early 2006.