Several dental school-related websites offer interview tips. In addition, many bookstores and libraries carry books to help you prepare for your dental school interview and help you understand the psychology of the encounter. Reading prep materials will help you ease your nerves and will lead to a more natural interview.
If you had a period of less-than-stellar grades, go into your interview expecting questions about it. Your focus should be to turn negatives into positives. For example, you can explain your lower grades by pointing to a heavy load of extracurricular activities.
The amount of time that it will take you to adequately prepare for your dental school interview is subjective. You will need to devote enough time to conduct practice interviews, familiarize yourself with the specific school's program, and read any prep materials that you have obtained.