Is there a rule of thumb for defining the scope and depth of the Related Work section in a thesis?
The rule is to provide more details on those works that are most relevant to yours.  This is especially critical for those that seem to overlap with your work, or conceptually share similarities.  The most relevant ones are those whereby the problem is exactly the same, but your approach and others are different.  That means you need to cover these other approaches in detail.

For other not so relevant, you need to show you are aware of them.  Basically, you state, 'there are also works on blah, blah2, blah3.  However, this work is different because of x, y and z'.   Note that you go as far as providing the reference, and differences.  You might briefly state their aim(s).  

Remember, one key reason for a  literature review is to provide evidence that your work fills gaps in the literature.  So present whatever is needed to proof this point.