Once opened, how long will yogurt remain safe and tasty to use?

This depends a bit on the manufacturer.    Dannon claims their products retain their quality for  2-3 days after being opened, if kept sealed and refrigerated.   Stonyfield's organic yogurts can still be used for 7-10 days after the container is opened.  The National Dairy Council states that yogurt will keep for about a week in the refrigerator once it's been opened.  
 
Source(s): 
Dannon recorded consumer help line:  1-877-DANNONUS
Interview with Dannon consumer relations representative.  (name not recorded)
Stonyfield Farms recorded consumer help line:   1-800-PRO-COWS (776-2697)
Interview with Stonyfield Farms consumer relations representative. (name not recorded)
National Dairy Council Yogurt Factsheet PDF