Like chains, the common misconception of 'stretching' doesn't apply to cables. Bowden cables don't stretch under normal circumstances (excepting broken strands or excessive loads, the latter of which cannot occur on a bicycle)--instead, what is happening is the housing burying itself farther into the ferrules. Once the ferrules are seated all the way, the adjustment will stay...typically a handful of uses of the component. Grab the derailleur in its most relaxed position, and use the paddle of the shifter to pull the cable harder than a normal shift. If the adjustment doesn't change, you're done, barring any damage to the housing on the bicycle.