I would recommond taking a practice NBME at the NBME website. They can be purchased for $50-60. This version of the test is a score out of 800. you can find a conversion table for comparative scores to the NBME CBSE (2digit score out of 100- this is test version we take) and the USMLE step 1 (3 digit score out of 300 - med students take this test), since they are essentially the same test, just different lengths. The NBMEs are 184-200Qs long and the STep 1 is currently 308Qs (I believe). You should be shooting for a 65-70+ score on your NBME to be competitive. Use Pathoma (go over multiple times), FirstAid for the USMLE Step 1 and UWorld qbank. If you want to start with an easier q bank, use UMSLERx for a little but UWorld is the ultimate tool. Expect Qbank averages below 50% when you start and maybe 60-70% toward the end. At your stage in the game, if I were you, I would plan to study at least 4-5 months solid in the evenings with a dedicated 2-3 week portion before the test.
I am in the same position as you (except I'm only about 2-3 yrs out). Depending on your grades from school and also even more likely since you have been so far removed from dental school, you will likely have to do a 1 year internship, as I predict that I will have to do.