Let's start with Undergraduate.  For many students this is called "premed".  Thank goodness I am done here.  For MOST people that go to medical school, they start out by getting a degree in one of the sciences (most often, biological sciences).  If they don't get a degree in one of the sciences then they still have to take a number of science prerequisites to get into medical school.  The whole process for MOST students takes about 5 years.  Some students are able to complete this requirement within 4 years and others a lot longer.

Then there is medical school.  This is where I'm at.  Technically, medical school takes 4 years.  the first two years take place in the classroom and labs.  The second two years are in the hospital (called rotations because we rotate through the various areas of the hospital).  

Then there is residency.  This is where it becomes more complicated.  During residency, we are not in school anymore and we are called "doctors", but we are still learning and are getting paid very little.  This is also where there is a lot of variation.  Depending on the residency, it can take as few as three years to complete and I honestly don't know how long the longest residency takes.  After residency has been completed, one can become a full fledged, practicing doctor or one can continue their education with a fellowship.

Fellowships allow doctors to gain extra experience in a particular area.  Many times, this is how a doctor "specializes".  I am not sure about the various fellowships and their time commitments.  

So, in total, it takes 4 years to get through medical school.  For primary care doctors it takes about 7 years before they are finished with their training but I think it took 13 years for Emmy's cardiologist, to get through his training in medical school, residency and fellowship.  As you can see, there is a lot of variation and in either case, it takes "a long time".