Quick answer: probably shorter than you think.
Below are a variety of AP Physics 1 and 2 prompts, and how I would suggest structuring your answer. Please note that, while I do grade the exams every year, I am writing here in the role of independent observer. I am not a representative of the College Board. These are my own simplified instructions to my own students, which may not be perfect in all situations.
Yet I think teaching is much better done by simple guidelines rather than legalisms. If you want the legalisms, go to the College Board's course and exam description; and look at the AP Central page where they go in to great detail about the requirements for a paragraph response. I don't think a student wants to see such detail. I think, in fact, that we should actively discourage students from a rules-bound approach to any answers. Encourage your students to simply answer each question, and move on.
On my last day of class before the exam, I will remind students of the types of prompts below, and my guidelines for the length of the response required. And then I will let go, and wish them the best.
"Briefly Explain:" or "Briefly justify" Answer in one sentence.
"Derive an expression:" Use variables only, start with an equation from the equation sheet or a fundamental principle. It will help to annotate your work with words, but complete sentences are not necessary. Full credit can usually be earned without words at all as long as the mathematics is communicated clearly.
"Describe a procedure:" Two to three sentences, never more. Say what you will measure, and what equipment you'll use to measure it. And stop writing.
"Explain" or "Justify your answer:" About two sentences, or perhaps one sentence with reference to an equation.
"Answer in a clear, coherent, paragraph-length response:" Five sentences, four is often enough, especially in physics 1. Physics 2 might need six or seven sentences. Do not repeat the question in the answer. Get to the point.
For all responses except mathematical derivations: Use sentences with subjects and verbs, but without fluff.
What if I need more space than the question provides? Then you are writing too much. The amount of space provided is deliberate, and reflects the length of response expected. Yes, I know you are allowed to use scratch paper and staple it into the book. You're also allowed to publish your bank account information online. Just don't.
Don't fear for the lost point. Students tend to write page-long essays because they fear that the grader will "take off" if they miss one small detail. But those students miss the bigger picture. Running out of time on question 5 could cost them seven points, while writing an extra page may in their imaginations earn one point. And writing that extra page is far more likely to lose credit for an incorrect statement than to gain credit.
Please don't be afraid. Answer each question briefly and confidently. If you have to guess, guess briefly. Believe it or not, we readers know when you're just writing random crap because you have no idea how to approach a problem. And rubrics are written such that they are unlikely to award credit for baloney.
Kick arse on the exam. Let me know how it goes. I'll continue to post my solutions (at least to AP1) each year.