If the force is proportional to velocity.

F= -b*v
From Newton's Law:
F= m dv/dt =-b*v
Then, dv/dt=-(b/m)*v

If the velocity V(t=0)=V0
Then the solution for the above differential equation is
V(t)=V0*e-bt/m

The velocity will not reach zero from mathematic point of view.
But it's magnitude will reduce to nearly 2% of its original value by the same time step t=4*(m/b) seconds.
if m/b=1s, V0=100m/s,  (the following values are estimated values, use exponential function to calculate it's exact value.)
at t=4s, V0=100*0.02=2m/s,
at t=8s, v0=2*0.02=0.04m/s,
at t=8s, v0=0.04*0.02=0.0008m/s
...