It is all so variable as 9' is standard with a dryline but when it comes to fishing sinktip's it all depends on what length and speed tip you are fishing and what the depth and water speed is and even where the fishing are holding.
I generally fish 4-5' leaders when fishing tips. By changing your leader length you can vary the depth at which your fly fishes.
If you fish long leaders on a deeply sunk tip and are fishing bouyant flies you will be fishing higher up in the water column then fishing that same fly on that same tip with a shorter leader.
Now if you shorten up the leader and fish that same tip and that same fly you will be fishing much deeper because that fly on the short leader will be carried down much faster andnot float up from the tip.
Now if you are fishing a heavily weighted fly, you can fish a ligher tip if you keep your leader longer. The fly will sink very fast and stay down.
Just keep in my mind that the standard heavily dressed marabous do not sink well at all as do not some GP's and hairwings dressed in a certain manner.
Hope that helps!!
Ryan S. Petzold
aka Sparkey and/or Special