1. The store is opened for 24 hours.
When the 24 hours are over, the store will be closed, since 24 hours is how long it will be open for.
2. The store is opened 24 hours.
The store never closes.
Number two isn't quite right, though. It should be -
The store is open 24 hours.
Or, better yet -
2. The store is open 24 hours a day.