The college application process is long and time intensive. A general rule of thumb regarding college admissions is "the sooner you begin, the better." Over the past decade, colleges have been making their admission choices earlier and earlier. For practical reasons, it is a good idea to have your applications ready for submission no later that Thanksgiving of your senior year.
Over 600 colleges use the Common Application for college admissions. The Common App allows students to create a single application to send to multiple colleges, eliminating the need to compose a seperate application for each school. Students appling to Common App schools should begin their applications during the Fall of their senior year, as all data on the site is erased at the beginning of Summer.