If it is a class 4 then it might take more than 15 min , so if ur gonna use ur sd card as ur primary storage then u might want to upgrade to class 10 , that's my choice. I myself using 32gb SanDisk class 10
Hard to say but your card is not that big so it should not take more than 15-30 minutes.
Sometimes the phone can not find/fix the errors. In this case you can get the SD card out of your phone and put it in the card reader of your pc and check it.