It depends... If you've got your own practice, then you make your own times. If you're head of the research project then you've got free range of times. Or you could be a doctor working 60 hours a week at a hospital or as a research slave under an abusive project leader.
There are many possible schedules that come with being an MD, and this is no different for physician scientists.
The 80/20 split you mention can look very different depending on who is doing it. Perhaps you research a rare genetic disorder and you choose to see patients with that disorder at clinic once or twice a week. Maybe you specialize in a tropical disease and you only travel to do clinical work for a month each year. You could even decide to work clinically only with techniques that directly parallel your research - for example, taking EMG recordings on patients and then working on modeling software that uses such recordings to predict disease progression. Each of these paths will have a different impact on the physician's family.
As far as research in general is concerned, it should be noted that once you run your own lab you will not be the person awake at 1am waiting to finish an experiment. You will be the person responsible to writing grants, traveling to present research, and checking in on your colleagues during lab meetings. These things, unlike long nights on call, have a little bit of flexibility built into them. Have an important family engagement? You can always fly to a conference a day late. At the end of the day, it just depends on the person.
As a PI, while you have more demands on your time there's a lot more flexibility as well. My current PI (MD who works on a genetic disorder) is always gone by 6-6:30pm to have dinner with his family. He does end up coming back in later sometimes, or usually works from home, but he isn't tied to the hospital/lab and unable to see his family. Same with my previous PI (MD/PhD working on development), he would fairly frequently go out of town with his kids but would take his laptop and work from the road. And my father (MD/PhD in anesthesiology but he lives abroad) would usually stop by my town on his way to or from conferences every few months, something he wouldn't be able to do without the research part of his career.
I will say that one common trait I've noticed is that physician PIs don't sleep a lot, so if you like sleeping more than 3hours a day it might not be for you, though residency might change that.