Models & Qualitative Behavior · Challenge problem

At high doses the liver's ability to clear a drug saturates, and the blood concentration $C$ (in mg/L) follows the Michaelis-Menten model $C' = -\dfrac{C}{1 + C}$, with $t$ in hours. How many hours does it take the concentration to fall from $4$ mg/L to $1$ mg/L? Give an exact answer.