I did not read everyone's messages. I'm from the west coast, where drop shotting got its start. I do not use a swivel. I tie my lines together with an improved Albright knot. I like it because its compact, casts through your guides very well, and when its tied properly, doesn't break off easily. You didn't ask about that, but there's some free advice.
As far as your question about how long to tie the leader, I typically make that based on water clarity, time of year, how deep I'm fishing the drop shot, and if there's grass. If I'm fishing in inches, not feet, I tie it the length of the lure. If I'm fishing deeper water with grass (typically means muddy bottoms and water conditions in my experience) I tie the rig so the lure is just above the grass. If I'm fishing chocolate water, I don't use a drop shot. I opt for moving baits at that point. If I'm tired of drop shots and want something different, I power shot. Heavy line with a jig on bottom instead of a weight, and then I tie my drop shot 3-4 feet above that. Hope this helps.