Most dogs will maintain their claws at the correct length whilst excercising on firm surfaces. If the claws become too long they may need clipping.
As a rule of thumb the claw should end on a line projected straight out of line of the pad. If the claw goes much past this line clipping will be needed.
There is a blood vessel in the middle of the claw and care must be taken to avoid this. In whit claws the level of the blood vessel is usually obvious but in dark claws an estimation of the position is required before clipping the claw.