1. 6 years - do a fake PhD where you are held to a lesser standard than normal grad students. This happens at some places. Or don't do a bench-science project, other disciplines may be more suited to an earlier exit with still a quality PhD. Otherwise forget about it.
2. 7 years - the committee advice is all fine and dandy, and you gotta work crazy hours in the lab yada yada yada but let me let you in on the real key to getting out in 7 with a real quality PhD:tons of arbitrary luck.
If you are dead set on getting out as fast as possible here is what I would recommend:
1. choose your rotations wisely and do a real/quality rotation prior to starting MS1 (the sooner you choose a lab, the sooner you start learning techniques/generating data for your dissertation).
2. choose your MENTOR and your COMMITTEE very very wisely (talk with current grad students and MS3/MS4s in order to avoid the bad apples who might hold on to you too long). look at the track record of the PIs you are thinking about working with as far as producing quality PhDs in a reasonable amount of time.
3. let your advisor and your committee know right away what your projected timeline is (i.e. I am planning to go back to med school June 20XX if everything goes according to plan) and remind them of that. Obviously you must be flexible if your data is not up to par, but it never hurts to plant that seed that as of June 20XX you are as good as gone.
4. meet w/your committee q6mo and make yer data SHINE so your committee will feel good about you graduating in 3-4 years. lay out your data as these 6 figs are paper 1, the next 6 figs are paper 2, and the upcoming experiments will be paper 3, etc. meet with your advisor often, especially if you have a laissez faire type PI, let him/her help you avoid the traps/bombs you might walk into that waste time/energy.
5. do not waste time as a grad student trying to keep up your clinical skills/knowledge by volunteering at health clinics, etc. there is a time for that - it's called med school. when you are a grad student you have one purpose in life, to learn how to think like a scientist and generate publishable data. anything that detracts from that, no matter how worthwhile you think it is (in my opinion, totally worthless to do clinical stuff as a grad student) will only add time to your grad years.
6. work hard. typically pulled about 60-70h per week, which doesn't seem so bad. many weeknights and Fri/Sat nights were spent there too, which is a necessary evil. and be productive in the lab - don't be surfing on sd.net while your gel is running, go clone something, set up a transfection, etc. there is a lot of down time in science that can either be wasted or used productively. move your project forward every day, even if it is only by the smallest step.
7. don't wuss out. it will be torture, but get used to that, because that is the life you have signed up for. don't let that affect your work ethic. keep grinding away and you will make it. realize that you get more and more productive the longer you are in grad school - 6mo of work at the end of grad school generates way more data than 6mo at the beginning. the data/productivity ~should increase logarithmically in relation to time spent in grad school. if it doesn't, well you can always forget that research career, go back to med school and become a dermatologist and make 390,000K while working <40h a week according to the NY times. oh, the horror.
8. finally, find out when you want to go back to med school. i recommend starting back at the same time all the other 3rd years do (July). you are already at a huge disadvantage (at least i thought it was a disadvantage) compared to the other 3rd years who are fresh off the boards, and your clinical grades (which are absolutely huge in residency apps) are often heavily influenced by how you did compared to your peers on a particular rotation. now, if you plan to go back in July, set up your defense in april or may b/c INEVITABLY it will get pushed back for one reason or another. realize that all committee members need to be there - get with them way in advance to make sure no one is going out of the country, etc. it is almost impossible to get 5 PIs in one room at the same time for a 2h stretch. defend in april/early may, hang out in the lab for a month tying up loose ends, revising papers etc. and then YOU ABSOLUTELY MUST TAKE TIME OFF BEFORE 3RD YEAR. at least 2 weeks. also realize that as a 3rd year you will have almost no time to work on papers that come back from reviews. try to plan for this by organizing your reagents/notebooks in such a way that a tech can do the necessary experiments.
9. endnote or reference manager - learn to use it, learn to love it. file every paper you read into it. when it comes time to write your papers, thesis, etc. you will think it is the greatest thing since sliced bread. read one paper per day M-F for the 1st 2 years. thats 500 papers. you will know the lit early on, be able to design experiments better, etc.
10. my final advice would be to get out while you still can, the shackles are not yet on. you have no idea what you are getting into.