It depends on the variety of St Augustine. If you are talking Floratam, I try to cut it at 3 1/2" in the spring and then move to 4" in late spring and 4 1/2" by early to mid summer. It seems to stress the grass a lot less if it is cut high in the summer. Some lawns that are somewhat shady can be cut at 4" through the summer with no adverse effects. Other varieties can be cut lower.