How much time would you recommend between the courses? I've heard of a few "Zero-to-Hero" courses . . .
I don't think it's possible to give a recommended time. It's no so much about time as it is about doing dives and getting comfortable. You can wait two years between courses, but if you're not diving in between, what's the point?
I did the Zero-to-Hero course. I never had a real intention of diving caves, I did it for the skillset. But once I was there, I was hooked. I don't mind saying that I struggled with it either, especially the first few days. My instructor told me after the first day of skills that we would probably just finish the cavern portion and call it quits. By the end of the second day I was showing drastic improvements, so we continued on. By the end of the class I was tired both mentally and physically and felt like I'd been in a car wreck. But I was also relaxed and comfortable and we were able to get in all the dives, all the skills and even had time to do some "fun" dives before I left.
Three of my buddies also elected to take overhead training after me, but all of them chose to break it up. That was mostly due to time constraints, but all of them said they doubt they would have been up for taking it straight through. One stopped after completing cavern, one is intro and my main dive buddy went on to take full cave. But I accompanied each of them as their buddy during training so it was almost like a refresher course for me each time.
It really, really boils down to the individual and to the instructor teaching the class.