Wireguy,
No, there aren't any real 'standards'.
The leader is just a tool, like any other portion of the rig. You choose it based on how you want it to perform.
Generally:
Shorter in stained to muddy water, longer in clear.
Shorter for a sinking line, longer for a floating.
Shorter in heavy cover.
You can use a floating line and long leader to get a weighted fly down to the fish (or actually, a long tippet).
A heavy butt section with a faster taper will help turn over bulky wind resistant flies.
But, what usually happens, you use what you have...
Good Luck!
Buddy
It Just Doesn't Matter....