How long can you stay in the US after e visa expires?

As long as you can get away with it. Legally you are a criminal as soon as your visa expires and you are still here, unless you have one of any number of legal excuses (i.e. you lost your passport, you are sick etc.) that INS recognizes. In fact there are a lot more illegal immigrants than there are INS agents and (the 12 to 20 million estimated illegal aliens currently in the U.S. sort of bears this out) unless you draw attention to yourself you will almost certainly not be caught.