Stop when you're done desribing, when somethng else moves in don't push that aside to keep describing the other object. Start descibing whatever new shows up. Boredom is a sign that you've done enough, as is the theme repeating itself. Try setting a timer for 10 minutes describe what you can in that time limit. It gives you a specific stopping point whether you are finished or not.
2) Just pick a point. What really catches the minds eye in that scene and go into the deatils if rusty nails and splinters become the focal point just keep going with that. You don't miss anything by having moved into details of a specific image. Remember if you have strong feeling like overwhelm from the amount of images you could be describing... start describing that feeling of overwhelm.
Alex
