Haven't had to in a while, but the last time I did you take the bottles to the Meadows, the store in the middle of the campground and drop them off there. Typically they are ready by the next day, but when I took mine in the morning it was ready by that evening. If I remember correctly, if it's dropped off by 12 it'll typically be ready by 2:30. They take the bottle to another location to fill it, so it won't be filled at the time you take it.
Haven't heard that this has changed any.