Time management as a graduate student is really hard. It’s hard to do things like calendar effectively, set deadlines, manage your time effectively when it’s for a kind of project you’ve never done. Even if you are in a program that is ethical as far as time off, it’s hard to figure out how to use that time for a few reasons.
First, far too many faculty endorse toxic notions about how much people should be working, and advocate irresponsible and unethical relationships to work, talking like we’re a gamer startup or high-powered law firm, and should be grateful to get an afternoon off every couple of weeks. Those people get paid a lot more than graduate students (or faculty) do, and just because there are fields that are unethical and exploitative doesn’t mean we should be.
Not only is that model unethical, it’s unsustainable. The little research there is suggests that people who thrive in academia don’t work sixty hour weeks, sacrifice any life other than work. They make strategic decisions about their time (including deciding to do some things badly).
So, one thing that makes time management as a graduate vexed is that people give bad advice about it.
Second, graduate students were excellent undergraduates, and undergraduates are actively rewarded for having shitty time management practices. It’s conventional in time management to use a process that, I’m told, Eisenhower made famous (but Covey has written a lot about it): thinking about tasks in terms of urgent versus important. In terms of the lives of graduate students it looks like this. 
It’s generally considered bad time management to spend most of your time dealing with tasks that are urgent and important and to ignore important but not-urgent tasks till they become urgent, but that’s what undergraduates have to do, and it’s what graduate students have to do while in coursework.
Third, (or maybe this is really part of the first), far too many graduate advisors tell their students they have to do all the things, and do them all beautifully, rather than teaching students how to be strategic about choices. It’s important to understand that faculty, especially in the humanities, are in a terrible position ethically. But that’s a different post. The short version is that a lot of faculty can’t deal with the cognitive dissonance of wanting to have a lot of graduate students (so that we can teach graduate classes, which are hella fun) and the fact that those students are going into debt to get a degree that won’t get them a job. And they resolve that dissonance by telling students that “if you get a magic feather, you will be fine.”
There is a fourth problem, true even in programs with good placement. There are no good studies on the issue of scholarly productivity, as far as I can tell, and that absence of research means that it’s a problem to give specific advice about how much time a person can spend a day writing. Many ethical programs give graduate students a teaching-free semester for completing their dissertations, and I completely support that effort. As I said, no studies to support what I’m saying, but I’ve consistently found that it’s hard for anyone to write more than 3-4 hours a day. In my experience (and I tracked this pretty carefully), writing for 3-4 hours a day in the morning (with breaks) enables about 90 minutes of editing in the afternoon. Graduate students, even ones on fellowship, often feel that they should be writing their dissertation eight hours a day, but I don’t think that’s possible.
The fifth problem is that faculty are too often dogmatic that graduate students must follow a writing process that isn’t actually working for the faculty members insisting on a process. Throughout my career, and at every institution, there have been faculty with wicked bad writing blocks–who haven’t published in years– who insist that students follow the writing process that is clearly not working for them.
My point is that time management as a graduate student is vexed because there are institutional restraints (including, possibly, an advisor with toxic notions about work and writing processes) such that much advice that graduate students are likely to be given is useless.
So, what is my advice for graduate students?
Calendar back from your deadlines, don’t expect to write for more than four hours a day, find your best four hours (which for a lot of people is ridiculously early), have at least one day a week and at least a couple of hours every day when you feed your soul—walk, run, play basketball, hang out with beings you like (and don’t talk about your work), do yoga, cook something interesting, garden, read shitty novels.