Geneva,
Why would you EVER want to say to students without disabilities, "Oh, by the way -- I am giving John more time than I am giving you, but it's only fair because he has a disability"? I am assuming you would not say it in those terms, but to suggest that you are going to "let the students know up front that they have a certain amount of time" but that others will have MORE time is not good practice.
Dr. Jane Jarrow