it really is variable by institution, and sometimes even by department and the role you play within that department.
in general though, institutions have their own salaries for what they think their docs should earn -- not as much as what you would get in private practice, but not crying poverty either. it is usually based on the assumption that they will do X number of hours of clinical work/week, +/- Y hours teaching [residents, medical students] and Z hours research.
so you get your salary, and you usually start of with a susbtantial portion of your time being patient care--you are generating revenue for the hospital (in exchange for a guaranteed salary). at the same time you're (probably) working on obtaining some grants. once you have some grants you can "buy your time", that is, say I want to be in the lab 3 hours a week, my grant would cover that portion of my clinical time. (hospitals or universities usually ask for some ridiculous amount of money in 'indirect costs' to be covered by the grant -- i think it's partly to offsest lost clinical revenue).
as you increase your grants, you can decrease your time in patient care (if your chair lets you).
if you aren't really doing research but more teaching, then usually in your agreement you establish how much time you'll teach over the year, but in general you end up spending a lot of time in patient care.
