The goal is to position your elbows at an angle that is comfortable for your shoulders -- one you can sustain throughout your entire task. Not only will elbow angle vary according to your mower's handles, but it will also be influenced by your upper back, neck and shoulder posture. Experiment to find a position that goes easy on your shoulder joint, yet is efficient. Also, take note of the positions that are not so comfortable so you can hone your choices.
One you have your working angle, keep the tone in your arms -- this will help you maintain your elbow positioning the entire time you mow the lawn.
