Nobody reads treatments, so unless you're writing it on assignment - don't (unless it's for your own resource - which in that case "standard" is immaterial).
There is no industry standard. If this is a work-for-hire, I'd simply ask the producer or exec what he expects.
Well, just for the sake of clarity -- nobody reads treatments for *spec* screenplays.
A treatment is very commonly a *step* in a script assignment -- execs want to see a treatment (or to be precise, they want to see a treatment, then another treatment, then another treatment -- and maybe then another treatment) -- so that they get to comment and misunderstand exactly what the movie is you intend to write for a few weeks to a month and a half before they finally let you move on to the first draft.
As to what they "expect" from the treatment -- in my experience, execs and producers who read treatments expect exactly the same thing that they expect when they look at rough cuts.
They *expect* to be able to see what the finished work will look like -- and since they never can, they get all worried, excited and upset and start giving you tons of notes and asking you to change things, hoping desperately that maybe the next version -- of the treatment -- will somehow satisfy that "itch."
Because just like everybody thinks they know how to drive and everybody thinks they know how to make love -- every exec and every producer believes that have these dual skills -- to be able to read a treatment and know what the finished script will be like and to look at a rough cut and know what the finished movie will be like.
But as with the former two things -- very few people can really do either of these things really well. Many can't do them at all.
Treatments for comedies aren't funny. Treatments for horror movies aren't scary. You always hear the notes -- "the characters aren't fully developed" -- Well, gee. Here we have a ten page document that's giving you an outline for what's going to be a hundred and ten page document.
Nothing is fully developed.
In fact, were you to have something fully developed at the level of a treatment would mean that you were doing something wrong.
That's why any story that has even a slightly complicated or convoluted plot is not only extremely difficult to pitch -- but even to write out as a treatment and try to get past execs is murder -- because summarizing things like scenes bouncing back and forth in time or multiple story lines -- even as you're writing it, you can just sense their eyes glazing over - their "not getting it."
Well, anyway -- I fear I've run past the original poster's question into semi-autobiographical material.
Carry on, all.
NMS