Our breeder told us that ears perk up between 8 and 12 weeks. Sometimes earlier. Nutmeg had floppy ears when we picked her up at 8 weeks. About a week later one ear went up. About a week later, the other ear went up. The one ear up/one ear down phase was very cute. Be sure to take lots of pictures as your pup goes through the perking up of the ears. You'll want them later.
One of my corgi's always had her ears straight up but my fluffy's ears didn't go up until she was about 3 months old. Even now, she still keeps them down about 30% of the time
At around 11-12 weeks, they sort of just popped up overnight :D