Many personal trainers get you to do 20 sets per muscle mass, they literally kill you while training. But the question is is it worth it.
Dr Rhea did a extensive review of this issue and found this (single set vs multiple set study)
Set needed per muscle group:
beginner 3 sets
trained 5 sets
Athlete 8 sets
So to make it is easy for average people training chest, shoulders and triceps would be like this: 6 sets of chest, 5 of shoulder and 3 of tricep with 30 seconds of rest in between for a total of about 20 minutes of training will give you similar results as a killing workout.
How much protein to take. For this you might take a look at Dr Peter Lemon
Strength athletes: 1.7-1.8 grams of protein per kilogram of body mass per day (or 0.77-0.82 grams of protein per pound of body mass per day)
Most bodybuilder take 1 grams per pound per day.
Which protein to use: make sure is low in fat, low in purine, low in cholesterol. Isolate whey protein is in the zone.
Sun will be good for the health in general and your muscle and bone muscle, Frank Zane used to talk about it. But for sun issues Dr Hollick is the man: http://www.youtube.com/watch?v=Cq1t9WqOD-0. Just enjoy the beach.
Good news: maintaining muscle mass doesnt take extra protein intakes, something close to a normal diet could do the work.
Muscle is needed to avoid injuries, to look better, to increase you brain health(proven in Dr Amen studies), for your overall health, to have more stamina...to excel at sports...list go on.
Please note that all said is supported by well known and highly respected authors. I know it is probably not the best system but it is really easy and little time to be done which increase the chances to stick to it. My training partner does the 20 sets system and his arm are one inch smaller that mine besides I'm overall stronger that him except for a few muscles. I do 2 or 3 innitial sets for warming up with very low weight not included in the example given.