How long will it take for my shoulder to heal.( 2 more questions involved) 10 points for best answer.?

I was working out and my arm started hurting.last time it was the left one which happened 3 or 4 months ago. why does this keep happening i stretch and warm up and cool down. i work out my chest and arms on mon. and thur. and my abs and legs on tue. and fri. i rest on wed. , sat., and sun.,but i run for 20-25 minutes on sat. and sun.If i rest will i lose muscle, because i heard you don't start losing muscle till you are 19 or 20.also is my workout two long (flat, incline, and decline bench press, incline dumbell flies,wrist and bisep curls, and pushups and dumbell shrugs < 3 sets each =about an hour and 10 minutes> ).oh and i take 2 minute breaks between bench presses and 45 seconds to 0ne minute on the rest.