I came across a great recommendation on the “Phinished” website to try rotating ways of working. 15 minutes a day is great for a while. Then it’s often helpful to switch to, say, a goal of 2 pages a day for a while, and so on. See “Erika’s post”: “The 40 minute system and research on productivity (somewhat long),”
http://www.phinished.org/showthread.php?t=4184
For grad students—the “Phinished” website:
MF