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