How long does downwash last?
Like everything else, air will be accelerated either upwards or downwards if a force is applied to it. The aircraft's wing applies a downward force to the air mass thus accelerating it in that direction. However, nothing applies an upward force to that mass of air, so it can be assumed it will not climb back up. 
(I am neglecting the formation of vortexes and possible differences in air density)

The velocity of the mass of air will be reduced by friction with and the inertia of the surrounding air until it comes to rest.

What you call the "normal flow path" of the air is it simply being still before an aircraft flies through it.