I am a U.S. citizen and my wife is an international student who is currently studying in the U.S.   What paperwork do I need to file and how much do I have to pay to the CIS? \n\nCan she apply for a work permit too?\n\nYour help is greatly appreciated
Since you are married, you will be filing an adjustment of status (form I-485, $325 fee) (also you will file an affidavit of support, form I-864, $70 fee), which is the path to conditional permanent residency for your wife. It does not matter that your wife is a student or not.\n\nOnce she gains conditional permanent residency, she will have legal presence and will automatically have the ability to work. \n\nHowever, if she wants to work before gaining permanent residency, there is a process to apply for an earlier Employment Authorization Document.
The instructions for the form do allow for some leeway in the "how long do you plan to stay" question.  Still, you should call the office at your school that produced your I-20 form to make sure that they didn't make an error.  As you read, the I-20 specifically asks about the "normal" term of study.  If your school feels that 36 months is normal, put that on your DS 156.  But if you call them and they say that they made an error (which seems unlikely), then you should put whatever they tell you is the correct term of study.