hmmm.  i have to think about how to explain this in a way that isn't as cyclically defined as the awful manual.  draping is there to let clothes respond to gravity that don't load that way initially.   there's lots of reasons to model something this way, like wanting the model to drape nicely over things other than your main figure or the ground.  it's easy to build realistic draping against your figure and the ground into the model, but you couldn't really anticipate anything else in a scene. 

in this case, my guess is that you're starting with a T-pose and the dress kind of floating around the figure.  if i'm right, both the skirt and the sleeves aren't starting from strictly a realistic position.  well, that means any sim you do from that position won't be realistic.  so using some frames to drape the dress properly on the figure and in the scene will make the sim more realistic.

how many totally depends on the cloth settings you used, how the dress fits initially, and the results you want.  i couldn't even begin to guess just by seeing your render.  i'd say start at 20 frames, and adjust by 10 frames from that result.  unless that first test totally isn't enough, and then i'd say try another 20 frames from that.  if 40 frames of draping doesn't get you anywhere close to what you want, i'd say look at your cloth density and air resistance.