I am a DiveCon Instructor, now retired. But when I was teaching DiveCons, I preferred that they observe & assist in two full OW diver classes; classroom, pool and OW. But if that wasn't possible, then the equivalent of one full OW class. (ie. One class or 1/2 of one class and 1/2 of a second.) This wasn't just "free labor" for me. I considered it an integral part of the DiveCon class. I needed to see how the DiveCons would behave in a real class. I preferred 2 classes, as I said, to get their student-face-time up. It made better DiveCons and later better instructors.
My personal experience: It took me about 3 months to complete my DiveCon training and get my card. It was during the summer when lots of classes were being taught. I volunteered to work every class that I could and had a great time. It took about 3 more months to complete my ITC and evaluation and get my first instructor card.