6 years for me ... total
One other person in my program did it in 6, average was 7.5.
My advice?
Find a mentor that is results driven e.g. "a Ph.D. in my lab is 3 first author papers in ..... journals," then work hard ... like 80-100 hour weeks, keep multiple projects going, when one works stay with it till you have a figure. Write early and only show data to your committee that is accepted for publication or at least in final stages of peer review. Find this mentor (who will need to be well funded and hands off in the lab) in your first months of med school and devote at least 25% of yr1 and 2 to bench work. Do not join a lab where you have to work on other people's projects at first; you want the freedom to develop your own thesis from day one. Your mentor will have to like you, the only way to get a thesis committee to sign off on a short PhD is if you have the data AND your mentor pushes them.
With luck you can do it in 6, but even working this hard, best to plan on 7.
There are huge advantages to finishing early, not the least of which is the fact that your interests may change and every year counts in terms of flexibility to tailor future clinical and/or postdoctoral training.