Obviously, it's safer if you are able to prove sufficient savings for the entire program, but from what I remember, one year should be enough, as long as you are able to prove that you can or will get a steady source of income from somewhere once the one year has passed.
Although practically, as long as you have an I-20 that the school issued, it should mean that you are financially okay, and it's up to the consular officer assigned to your case to investigate further.
Edit: Just confirmed that you only need proof you can cover the expenses incurred in your first year + proof you will eventually have money/funds to support the rest of your studies.