With reqard to the "Date Needed" line, you want to indicate the latest time/date you need the material by. Chances are you want your book as soon as possible; however, putting in an earlier "Date Needed" will not necessarily expedite your request. As a matter of fact, it may be counterproductive because your request may "expire" before it can be filled. Allow the system at least two weeks. Standard operating procedure is usually to give the system thirty days.
A final word of caution - make sure the bibliographic record is the best record to use. A book may have both a British printing and an American printing. The bibliographic record for the British imprint may be owned by only a few libraries, while the American imprint may be owned by hundreds. Use the record with the greatest number of library holders. WorldCat recently improved its displays so that "better" records, at least from the point of view of library holders, are at the top of your list; however, it's best to scrutinize bibliographic records closely before bringing up the ILL form.