Hey! First of all, I recommend calibrating them at least once a month. The pH probe should be kept moist when stored in the cabinet by putting some storage solution inside the cap, which is usually designed to hold it. The EC probe should be cleaned with paper, while the pH one should be cleaned with cloth.
As for the levels to maintain, the pH depends on the substrate; in soil, 6.5 is great. For EC during the vegetative stage, I usually max out at 0.8/0.9, and around 1.5 during flowering. Sativa and autoflowering strains consume less.