“Nine hours of learning” or “9 hours of learning” are your best options here. The difference between using “nine” vs “9” is just formality, I think. For the most part, using either of them should be fine. With regards to the -, that typically works when you are saying something like “nine-hour-learning” without “of”. I’m not sure what the rule is of using the hyphen but I think it has something to do with forming complex/compound noun phrases. I hope this was helpful! :)

“Nine hours of learning” is formal. Generally in academic settings, numbers under 10 OR starting a sentence are spelled out. “9 hours of learning” is informal.

Officially, numbers less than 100 (I think) should be written alphabetically, but it's not that important. You can talk about a nine-hour learning session or a three-hour bus ride. You can also say, "With nine hours' learning, he was able to master the entire chapter." This means the same as "With nine hours of learning, ... " <-- this is probably easier and more common than the previous way of saying it.