Generally, viability doesn't go down much until after 2 weeks; however, how you keep them makes all the difference. The recommendation on the turning is a good one.
Here are a few other suggestions that will help them keep longer:
Keep them in a cool, dark place ( like a closet). (55 to 65 degrees F)
When in the egg carton, place the carton at an angle of about 35 degrees.
saladin