It depends on YOU and each school's MD/PhD program (because the programs can vary); many MD/PhD programs are "integrated."
In other words, some MSTP programs emphasize integration of the MD and PhD components of the training (e.g., graduate school courses in years 1 and 2) as well as clinical experiences during graduate school. Other programs allow completion of 3–12 months of clinical training before the start of full-time graduate training.
For each MD/PhD program in which you are interested, you should carefully read its MD/PhD website and/or ask about how the MD/PhD program is conducted at the school.
Here is a potential MD/PhD program timeline: 6-8 years MD/PhD program; followed by 3-6 years of residency (depending on your specialty); plus 3-6 years of a postdoctoral fellowship (depending on your field of study).
Is that what you want to do?