what ppm?

my plants are about a week old in a hydro set up (drip irrigation) using sensi grow 2 part solution, what ppm should i be at, and how often do i add more nutrients to the resovoir and how often do i raise the level of nutes? basically, how do i know when my plants are getting optimum nutrient levels?

Read the GrowFAQ, and ya might want to address questions like this to the Newbie Central on roi.