Good point but still interesting answers (thanks again guys).I don't think most people are reading your question right.
Lots of intel right there...IMO, there are a LOT of variables to this question. To simply say 3x or 2x the depth is a good rule of thumb, but sometimes that is not possible. When sight fishing, I prefer an in line indicator or no indicator at all. In line indicator being something that won't loop on to my line (thingamabobber is not an in line indicator). The little footballs work great for sight fishing.. tiny as can be. In these cases, I am usually using 5-6' of leader and the indicator will not be used as flotation for the flies.
In deep slow water, I want the flies low, so I use about 1-1.5x the depth I want to achieve. In fast water, it may take more, or less if the fish are high in the column.
Fly choice is also a factor... the length of leader I will use with a san juan worm (unweighted) and some added weight is actually different than the length that I would use with a heavily weighted pig sticker (worm pattern with lead underneath).
My point is this... there are standards that work great when you are starting, but paying attention to the subtle things that change the standard thinking is what will make you good. Depth is the single most important thing in nymph fishing and knowing how to adjust the depth of your rig will be your best tool at catching more fish... the fly pattern is about 1/10th as important as where it is presented.