How often should I run?

By run, I mean cardio, and by cardio I mean anything not lifting.  Cardio in some ways has characteristics to lifting, and in other ways none at all. 

When many do cardio, it's long & slow.  For that you can generally do it day in and day out without fail.  Your body
won't hate you (most of you) and early on you'll see progress, at least until you stop challenging yourself.  It's monotonous, mind-numbing, and yet can be oh-so-satisfying.  But you can do it every single day as long as your body says it's ok. 

Take interval training, or High Intensity Interval Training for you more technical savvy people.  This is often way more strenuous than your long & slow training (hence the "High Intensity" part).  Your ability to handle this with frequency is dependent on the action performed and even the duration of the activity. 

If you're running, you have to take into account the beating on your joints that will ensue when you do it often enough.  This means analyzing the terrain, the shoes
, the duration, and the speed of which you'll run.  If you're swimming, you have less (usually) to think about.  Generally speaking, running is simpler and easier to perform because we are all innate runners, at least in a limited capacity. 

If you were to do sprint running, at 100% every single day, it's likely your performance will suffer over time as you're just beating your body.  If you were to "cross-train" with say...rowing or swimming, you might be able to sustain 100% much longer and more frequently.  The same goes for long distance stuff, as you vary the intensity and type of training, it's much easier to sustain over time. 

The answer to "how often should I run?" is complex, as you were able to see.  Like lifting, it's very complicated and dependent on the individual and the choice of exercises.  In this case, variability is much more important to sustain a 100% effort level, but even then it's unlikely that you'll be able to sustain it long. 

So the best answer of how often is how often that you're able to.  That is always the correct answer, but know that the 100% effort is harder to come by.  Enjoy what you're doing and do it to the best of your ability.