1) Diacetyl rest is only needed if you actually smell or taste diacetyl after a few days into fermentation. If you try a sample after several days into fermentation and do not smell or taste any buttery or butterscotch notes, then it is safe to just leave it alone. I only get diacetyl on my lagers about half the time or less than half the time.
2) If you decide that you do need to do a diacetyl rest, you'll want to do it when the specific gravity points are about half of what you started with. So for example, for a 1.050 lager, you should try to start the d rest when specific gravity is at about 1.025 plus or minus a few points.
Number of days doesn't matter. The degree of fermentation according to specific gravity is what matters.