In terms of frequency:
Would 1-2 sets of about 10 reps twice a week be about right?
I do 2 sets of 25 reps at least twice a week with Therabands. Tie a knot in one end and trap it in a door etc. Cheap and i find much better than dumbbells as you can control the ROM much better and have constant tension on the muscles. As you fail you can move nearer the door to decrease the tension (or drop set). Equally move away to make it harder. Add more bands as required.
I added these to my routine thanks to NNM. I had a very noticable 'POP' coming from my left shoulder for months and now its almost gone.