This drag force is always opposite to the object's motion, and unlike friction between solid surfaces, the drag force increases as the object moves faster.
Assuming no other forces acting on the probe, according to Newton's 2nd law, we can find the acceleration attained by the probe due to the thrust (assumed constant) as follows:
Since the probe started from rest, we can express the final speed applying the definition of acceleration, as follows:
We can solve for t (in seconds) as follows:
Since 1 day = 86400 s, we can find the time in days as follows: