When a condenser discharges electricity, the instantaneous rate of change of the voltage is proportional to the voltage in the condenser.

Suppose you have a discharging condenser and the instantaneous rate of change of the voltage is $1/100$ of the voltage $($in volts per second$).$ How many seconds does it take for the voltage to decrease by $90\%?$

Please solve this using Mathematica $($the calculations must work when I evaluate the notebook$)$