I looked at the Box2D source code, where it applies damping:
v *= b2Clamp(1.0f - h * b->m_linearDamping, 0.0f, 1.0f);
So the loss of velocity is certainly not a fixed number of units per second, despite what the manual says.
Your example of 10m/s to 1m/s would also slightly depend on timestep.
Let's assume 120Hz simulation step...
v = 10
step = 0
h = 1/120.0
time = 0.0
while v > 1 :
v *= ( 1 - h * 0.5 )
step += 1
time += h
print( "step %d, time %f, v %f" % ( step, time, v ) )
Then it would take 4.6 seconds to slow down by that much.
step 551, time 4.591667, v 1.001957
step 552, time 4.600000, v 0.997783
I checked with 1/60 timestep as well, and the result is almost the same, btw.
I am not sure if you can determine this time delay in a single expression, though, without stepping through it.