Some more words of wisdom from Steve:
Virtually all golfers swing the club back further than they think. They will feel that the swing is short and then when I show them the swing it is significantly longer than they feel. There are few points I would like to make with regards to the length of your backswing.
As a starting point if you unable to control how far you swing back and therefore your awareness of the length of your backswing is not accurate, then your ability to control where your shots go and how far they go will be diminished.
Generally if you swing back to parallel or beyond your arms will disconnect from the turn of your body. This disconnect is very inefficient. You do not want your arms moving independently. Your arms a body should sync up, and then you can control the speed and length of your swing with the big muscles in your body not the smaller muscles in your arms and hands.
Some golfers will turn their body too much and this will allow too much backswing. The turning of your torso should create a torsional effect. Your legs will have hold of the ground and your torso is leveraging against your hips and legs. This position creates effortless power and control. If you are achieving this position it is virtually impossible to swing back too far.
Remember that golf is a game of repetition, the measure of how good a player you will become is attached to how often you can repeat a good golf swing. Controlling the length of your backswing is a big part of this process.