Your goal should be to make the biggest turn that's right for your body and flexibility level. Turn your hips and shoulders fully, and let your arms swing back to the point where you can keep your left arm reasonably straight.
If you hit a lot of slices and pulls, your swing probably cuts across the ball from out to in. A drill for this is to set up to the ball and make a practice swing as if you were playing to a target straight out from your chest--90 degrees from your actual target. This "sideways" swing will give you the sensation that your shoulders stay closed to the target into impact. After three or four swings, hit a normal shot and try to re-create that feel.