That would depend upon what you are casting. A standard extruded leader ending in 2X of 7 feet would be a good start, you can add 3X or more rarely 4X to the end if you wish. Some folks will just tie on a level leader/tippet of between 7-9 feet if they are throwing weight.
on a 10' 7wt rod i use a braided furled leader 6' or 7' long with 3' of 2x or 3x rio fluorocarbon tippet. I get the leaders from feather craft. love the leaders and use them on all my rods. for me i usually like a leader about the length of the rod.
I really like to tie hand tapered leaders 6ft to a tippet ring or micro swivel. Than usually for nymphing I like 3-4ft 5X orvis mirage. Also really like plain ole seaguar 100% in 4 and 6 pound test. If swinging streamers i use 10 pound mono for i don't find it to make much difference. Always has worked very good for me. BTW Tippet rings/ micro swivels rule cause when you break off no blood knots!