To reduce stress, I would recommend slow and steady running.
Certain forms of exercise (jogging,
cross country skiing, swimming,
hiking, bicycling) require a fairly
consistent repetitive motion that can
alter one's state of consciousness.
Described by some as moving
meditation, the physiological effects
of regular participation in these
activities is very similar to what
happens when one practices meditation.
Breathing and movement, act as a
mantra and may in part be responsible
for the feelings of calmness and
tranquility claimed by some in
response to exercise.
Taken from Exercise as a Stress Management Modality
You may also try to apply some of the recommendations from Runner's World where they summarize a list of stress symptoms and propose solutions to each.
Stress Symptom: YOU'RE DISTRACTED
THE SOLUTION Run on a flat, well-lit
surface, and try to pick a scenic
route you find calming.
Stress Symptom: YOU'RE FIRED UP
THE SOLUTION "Take it down a notch and
keep the pace comfortable,"
Stress Symptom: YOU'RE TOO TENSE
SOLUTION Improve your flexibility and
strength—especially in those weak,
Stress Symptom: YOU'RE EATING POORLY
THE SOLUTION Eating small, frequent
mini-meals that include a lean
protein, complex carbohydrate, and
monounsaturated fat helps stabilize
your blood sugar.
Note: Because you phrased your question as "how often and for how long I should run", this leaves out A LOT of other stress reducing activities, like yoga :)