The spark plug in an automobile engine is a R-L circuit as shown in figure. The circuit that provides the spark uses an inductor as the energy source. Initially switch is closed that allows current to build through the inductor. When the switch is open the current decreases rapidly through inductor and a large emf is induced by inductor. Given $\epsilon =12V,L=10mH,R_c=10\Omega ,R_p=7k\Omega$
If switch must be closed for up to three time constant, find this time in milliseconds