The length of time it takes to do a refresh is dependent on the number of entries or data it must read, not amount of memory used. Say you have two 4GB cards. One is filled with videos or movies, maybe 2 -3. The other card has 500 music files on it, but only 2 out of the 4GB are filled. the card with the music files is going to take a lot longer to refresh than the ‘fuller’ card with the 2 or 3 movies.
But the Class rating doesn’t really come into play in this refresh. It’s based on writing speed to the card, and the newer ones like the 16GB only come in Class 2. So I wouldn’t get all hung up on it.
For all intents & purposes, a Class 2 is just as good as a Class 6 in these devices. There have been numerous other discussions and nobody yet has been able to prove or disprove an advantage of one over the other in ‘real-life’ use with either music or video files.
Now maybe if it’s used in a digital camera where the images are being written to the card and you can’t snap another picture until it’s finished; that could be where the Class differences are more apparent.