Unless your expectation of grad school is 16 hour days work every day for 4 years, then it will be harder than you expect it to be :P In all seriousness, work will ebb and flow. Some weeks/months you will be inspired and working whenever you're not sleeping or eating. Some weeks/months/years, you'll be uninspired, spending a few hours checking email, news sites, then sit down and play video games or watch TV.

Really though, those days intermix, and the sum total of the work that I did for my degree (aside from coursework for the masters degree) could have been completed in 6-9 months. The rest of that time was spent (in order of most to least time) relaxing, trying to figure out what to work on, campus politics, working on things that didn't make it into any paper (but which expanded my knowledge in other directions), and waiting on other people.

Edit: forgot the requisite infographic http://matt.might.net/articles/phd-school-in-pictures/