So if the brakes are applied to a certain pressure and corresponding braking torque, they will maintain that braking until the actuators are driven in the opposite direction.
This is a bit oversimplified...
What brakes the aircraft (what produces a break torque) is the force compressing the brake disks (and the coefficient of friction). What is defining the force is the ammount of compression by the spindle and the stiffness of the brake disk package. If the brakes are active, they wear and become thinner (just a little bit, of course), but the also get hot, and with the negative thermal expansion of carbon fibre they get even thinner, while the housing from metal expands at the same time. Given the same ammount of deformation (spindle is fixed), the compressing force and hence the brake torque will fade with use. How much strongly depends on specific numbers. So you may lose a significant amount of brake torque if you do not adjust the spindles, which means that without electricity, you will probably end up with less braking power than desired.
which of course still is a simplistic view of the issue...