It depends on your level of experience. When you are just starting, doing an intermediate level case will get you two skill points and an easy case will get you one.
As you gain more experience, it will take more experience (cases) for each skill point. By the time you reach about 500,000 experience - it takes about 40,000 new experience points to get a skill point. So spend your earlier points wisely, because they come slowly later.
btw, I'm moving this to Newbies