Hi Anna,
I may be able to advise a little. Somewhere back on the threads is one "pain in the bum"! where it explains some of the aspects. With me, I pulled my piriformis by overcompensating for a slight niggle in the back of the knee. I found it would ease in a couple of days and I would run gently, but it would immediately hurt again. Not quite too bad to stop running, but bad enough to know that I should stop before making it worse. I had 4 sessions at the physio over 3 weeks. He had to get very deep into the painful area, if you know what I mean, and used his elbow to get enough pressure. He also used needles - not acupuncture - but to get to the trigger points to stop it recurring. If you are squeamish, best not to look at how far the needle has to go in!!!
Also several stretches for the piriformis and strengthening exercises for the glutes (which were being lazy, hence why the piriformis came into play).
How long?? I eventually had to rest it for 11 days (2hours and 24 minutes!) and then eased back into it, ensuring that I gently stretched it before running. Did 16 miles on Sunday, 6 weeks after it first flared up, and had little impact, although I did take it slowly. Went tonight for 2 miles and felt it, so, it is still not completely gone. Patience is necessary, which I am not blessed with where my running is concerned.
Hope this helps - let me know how you get on. John