Obviously, when the sensors can no longer detect the top of a wall (when there is one), there is too much skew error. However, the actual skew tolerance has more to do with how much space a robot needs to correct skew errors. When the robot is stationary, space (distance) is not a problem because differentially steered robots require no distance to rotate.
However, when a robot is traveling at a certain speed, it will take some distance before the error is corrected. Unlike offset errors, skew errors continue to accumulate offset until it is corrected.
The real limiting factor is how fast the motors can change speeds to steer a robot. The steering needs to factor in maximum linear acceleration/deceleration as well as angular acceleration.
There may be a closed mathematical form for this tolerance computation. However, I am not a mathematician. Instead, you can use a simulator to help you figure out exactly how much skew error your robot can suffer.