From my own experience and from what I have heard from DA, if you have a large task, a single NA that will take an hour or more to complete, then scheduling it into your calendar can be an option. You've got to write that employee evaluation and you just need to sit down for an hour and crank it out. Scheduling it can help ensure you get a contiguous period of time to get it done. I wonder, however, if "study" is a single NA? Wouldn't it be more likely composed of next actions like "Read Chapter 7 in Physics text", "Do Math homework due Friday", "Read pages 47-72 in 'The Diary of a Young Girl' by Anne Frank", and stuff like that? If that's true, then each of those would be a discrete NA. You could still block off time and label it "Study" to set an appointment with yourself to do these things, but it would be to crank out one or more of these discrete NAs. "Study" would not, in itself, be a Next Action.