If cast upon an object, the darkness spell causes that object to absorb
some light in a room attempting to make the room dark.  If invoked without
an <object>, the spells affects only the room in which it was cast.
   Plunging a dimly lit cavern should prove to be no problem, but only the
highest level casters will be capable of blotting out the noonday sun. The
effect lasts for <level of caster> hours.

      Darkness: -level

       4 hours + ((level * 1 hour) / 2)

      Level 10:
         Darkness: -10 light
         Duration: 4 + (10 / 2) = 6 hours

See Also: Spell Light