Pruning will give you more flowers over the entire plant.
It's best to prune after it's done flowering. Some people prune by half, others by a third: do what you feel is best. I prune to the ground in very early spring and have many blooms but the plant doesn't get tall, as yours does. It would overpower the little fence it's on if I allowed it to get huge.
You could prune half by half, and the remainder could be pruned more harshly or not as much.