Where did a response time of 12us come from?
The shutoff time is determined by how long it takes the voltage on C1 to drop from 6.8V to the threshold voltage of Q2, plus some delay associated with shutting off Q1. Clearly, the only thing that can remove charge from C1 is D1, the LMV431AIMF.

Neither the threshold voltage of Q2 nor the current capabilities of D1 are well-defined, so the delay is probably measured (empirical) rather than calculated (theoretical).

You can assume, however, that it will scale linearly over some range with the value of C1. At the low end, it will be limited by the gate charge of Q2. At the high end, it will be limited by the dissipation limits of D1.