Applying for an F-1 or M-1 student visa for study in the U.S. is not normally a lengthy process. Nevertheless, you will need to figure out two separate calendaring issues:
Your school will likely provide you with helpful information and support during the visa application process, but ultimately it will be up to you to deal with the U.S. government in applying for a U.S. student visa.
WARNING: The coronavirus pandemic changes all of the below. As of mid-2020 many U.S. schools and universities are closed, which makes applying for a visa potentially useless, or at least raises a question of when you'd be able to attend. U.S. consulates and embassies, which make decisions on student visas, are also in many cases closed to non-emergency visits. Expect future delays and changes in procedure.
If you are applying to academic programs (requiring an F-1 visa), it's normally best to start contacting schools at least a year before you plan to start your studies. The school year usually starts in August or September.
Competition for entry to schools in the United States can be fierce, especially if they are big-name schools like Harvard or Stanford. You will probably want to submit between five and ten applications to a mix of schools, including some that you know you have a good chance of being admitted to.
If you are planning to apply to a vocational program (requiring an M-1 visa), the calendar will not be so predictable. You will need to contact schools directly for information on admission and scheduling.
Only after the school has admitted you and issued a SEVIS Form I-20 can you take the next step and apply for your student visa. For guidance, see Student and Exchange Visitor Visas.
One of the risks you face is that you'll apply too late. The U.S. consulate might not issue your visa decision, or the visa itself, on the same day as you appear in person to apply.
In some cases, applications are referred for further administrative processing, which can take up to 60 days or more from the consular interview date to be resolved. Check with your local U.S. consulate for its suggestions on timing and application procedures. Many recommend applying three to six weeks in advance of your intended travel.
If you are already in the U.S. and in lawful status, and prefer to switch to student status without leaving for your home country, you can do so by submitting a form called an I-539. In that case, you will not be dealing with a U.S. consulate, but with U.S. Citizenship and Immigration Services (USCIS), in the United States.
You will need to check into how long the USCIS office that will handle your request is taking to process Forms I-539, for a change of status. To find this out, go to the Check Case Processing Times page of the USCIS website and review processing times for the California and Vermont Service Centers.
As you will see from USCIS's instructions for Form I-539 (in the "all other Forms I-539" category), you will submit your application to a “lockbox” in Texas.
When you receive your filing confirmation via U.S. mail, it will have your case number. Case numbers beginning with WAC are for California, while EAC refers to cases at the Vermont Service Center. Reviewing both Service Centers’ processing times in advance will give you an idea of how long it will take USCIS to process your I-539 application.
As of early 2020, USCIS was taking from three to five months to process change of status applications for F-1 and M-1 students at the California Service Center. The Vermont Service Center was slightly slower, at six to eight months. You would want to apply early enough to deal with this timeframe.