It varies. Seed viability depends upon several factors such as: temperatures in storage area, humidity in storage area, length of storage, genetics (i.e. how healthy were the parents?). Then your germination techniques come into play as well.
buy them from a reputable company, out of all my orders over the past 2 years from attitude ive only had one bean not germ. it was a barneys farm vanilla kush. other then that, everything else sprouted and went as planned. as far as fem seeds the only batch that ever went hermie on me was the mk ultra...