Rennet powder lasts basically indefinitely if stored in the freezer, from what I've been told. How much to add depends completely on its strength. Rennet powder can be *very* strong (for example 2000+ IMCU per gram compared to 200 IMCU per ml for single strength liquid rennet). Its strength is the main reason for not using it in the home -- it's basically impossible to measure out small enough amounts. Having said that, solitary_kidney on reddit has some powdered rennet that they buy in Greece which is only something like 80 IMCU per gram, which seems really awesome. Very easy to measure.