It depends on your mental ability to take in and digest new learning. Myself past my best years, I often found two one hour lessons on a single day too much to absorb to full profit, even if a couple of hours apart. I made best progress at a rate of three one-hour lessons per month, or so. If your mind is young and flexible, I daresay you could manage more.
But never forget that his exercise is expensive, and should be worth its money. When driving home from training, you absolutely must feel in great joy, or your expense was less than optimal.
The prime goal of training is to get ready for the qualifying tests - but the secondary goal of fun is essential, too.