The ideal time to begin your planning for graduate/professional school is at least one year before you plan to start a graduate program. (Current students: Summer before your senior year of college)
Everyone's timeline will vary based on field of interest, program and university, but this timeline can serve as a guide. Professional programs in health and medical fields may require applications to be submitted a year in advance of beginning a program.
REFERENCE: Kidwell, Clara Sue & Hamblin, Jane A. (1999). Graduate School and You: A guide for prospective graduate students. Washington, D.C.: Council of Graduate Schools.