Been on many different meds for severe pain for 7 years. Tried everything from meds, accupuncture, massage, physical therapy, tens units, 2 surgeries, and the list goes on and on. I was prescribed the MS Contin 15 mg every 12 hours. I took my first dose this morning. By this being an extended release, how long does it take To build up in your system to notice it working? I was told to still take the percocets as needed.