Focus on relaxing your lats in this position and begin to move your arms, replicating pressing and pulling movements. Try straight up, straight back, up and down, etc.
If there's a noticeable difference in either arm, see if you can start to balance it out. If you don't feel like you're getting much of a stretch and you have no pain, then you could try heavier plates.
As for times/reps etc., just do it until you have a noticeable improvement. Always have something to retest afterwards like your overhead press.
If you're a big guy and this gives you no improvement, you could also include a roller in the stretch to increase your thoracic mobility. Just treat this exercise with respect. It's a stretch, not a strength exercise.