Once the package is opened, the oxygen gets to the yeast cells and it's not good for very long. Maybe a week or so would be ok. I can't remember all of the science behind it, but it has to do with the package being sealed in an oxygen-less environment and when oxygen does get in through opening the package, it changes the environment of the dehydrated yeast. I think it was David_42 who explained it to me, but I'm really not sure.
Broken Leg Brewery
Giving beer a leg to stand on since 2006