Average of 6 years - just be sure to always pass your PT test, don't get arrested/do something stupid, complete an AEGD or a residency. Also, to make yourself more competitive (and possibly be promoted earlier - very hard to do this) try to deploy, go to various schools (EFMB, air assault, airborne, etc), be a brigade dentist, become board certified, stay on top of your professional military education (Captain's career course, etc)..