Foam Roller - How often?

Sometimes everyday, sometimes none.  I found the more often you do it, the easier it gets.  I usually hit each "spot" about 30 sec and then repeat.

Every day!  Every hour!  Every... well, ok, the last time I used mine was about 2 months ago, but if you're getting tight, once a day is a good idea - right after workouts.  Follow with an ice bath and your muscles will be thanking you for sure!

I ended up getting a foam rollar that I keep at work so I can get a couple of minutes in each day... (except I forget)

but when I am using it and some one walks in the look on their face as they turn away thinking that they walked in on me doing something "private" is priceless

Lately I foam roll a couple times a day for a few minutes at a time.  Have been having hamstring issues that are almost resovled (went for ART and GRASTON at the peak of it).  For the piraformis use a tennis ball and then follow up with foam roller.  Tennis ball gets into that area much better.

I was told by our club sponsor and local PT, try not to do it more then 3-4 times a week and your whole lower body should take around 8 minutes. You can over roll! Before I run, I do it the whole circumfrance of my upper leg for around 3 minutes per leg and a few seconds on the shins and calfs.

IMO rolling is a great warm-up before running. It pushes all the blood into you muscles.

I think depending on your training volume every day should be fine. When I was a personal trainer I would have my clients roll before our workouts, and stretch afterwards. If your piraformis is causing you problems after your run and stretch, a couple of min on the foam roller will be ok. You can also strengthen your piraformis, by standing feet about shoulder width apart, without moving your feet try and turn your feet out, it'll feel weird but it works.