A 100µF capacitor and a 15kΩ resistor are placed in series with a 9V battery
C = .1E-3 F
R = 15E3 Ω


τ = RC

(RC time constant)



https://www.electronics-tutorials.ws/rc/rc_1.html

I don't want to do all the work for this homework style question. Instead, here's an idea:


Find τ.