Is there a standard length?
No.
As far as end tippet material, I average between 2 and 3 ft, probably.
But a lot depends on how your leader is built, and the situation you're facing. If you have soft currents and you need a better drift, you lengthen the tippet. If that don't work, you lengthen AND go to a smaller diameter. If you are twisting up your tippet, or you are trying to squeeze a tight cast up under an overhanging branch without hanging up, then you shorten it, or make it thicker, or both.
And if your leaders are thick and stiff and "taper late", you probably need to average a longer tippet to get an appropriate drift. If you have a really soft leader, you may be able to scale back. Some also depends on your casting ability. If you can get the drift you need by altering your cast, that's better than lengthening the tippet. But if you can't, you lengthen the tippet.
This of course assuming you are trying to get a drag free drift. Sometimes I'll throw on an elk hair caddis or something and intentionally skitter the fly across the surface. That kind of thing calls for shorter tippets.
The real answer is "as long as you need it to be, but not longer".