Of course. Khatz himself admitted that he was rarely able to do so much, but he always tried.
We all have different amounts of time available for studying our target language. Immersing 18 hours a day is not always possible. I think trying to get to the goal every day is the most important thing though. If you do 12 hours, or 8 hours, or 6 hours, that's not a crime. Just try to do better the next day.