Whereas applications for F-1 visa may be submitted at any time, the visa can only be issued up to 120 days in advance of the course of study start date. If you apply earlier, the passport will be returned to you and must be submitted again at a later date. Students will not be allowed to enter the United States in F-1 status earlier than 30 days before the start date listed in the I-20 form.
Continuing F-1 students may renew their visas at any time, as long as they have maintained student status and their SEVIS records are current. Continuing students may enter the United States at any time before their classes start.