If you're doing 3-4 track sessions they're probably not that hard, not "killer" days. As always, relative to one's ability to run the times, and ability to recover.
There's no one answer to this question, since it depends among other things on, what period your training is in, what distance you're aiming to race at, and your development as a runner. Good discussion of this in Joe Rubio's chapter in Run Strong, and of course, in Daniels' Running Formula.
In a base phase, you probably want to be doing 3-5 hardish days a week. malmo, who posts here, would say that a natural tempo run is one you can repeat the next day. In terms of specific paces it's still above marathon pace, comfortably fast. Then as you get closer to goal races, easing off some to be tapered for races is the idea.
Possibly also, the shorter the race, the more hardish days you'll do. The two killer workouts is great for marathoning since it simulates what the race will be like.
And what people can handle will vary individually.