We had these labs as a base for our course on OS development. Original tasks seemed to be very good - even if you did something wrong, the tests would probably catch it. But those who made our course decided to introduce some changes, which led to the following proportions of spending time: about 0.5-1 hours for the task itself and continiously growing number of hours for debugging, first mistakes in their code, then the bugs we introduced ourselves. My first hacking experience, indeed.