I originally wrote about this in Big Time Chunks, Little Time Chinks way back in 2010:
Earlier this week, I tried adopting what seemed-to-me like an irresponsible attitude: I set a limit of one hour for up to two projects each day. The rest of the time, I would do whatever I wanted. The idea came from my friend Elise, who told me her method for practicing her flute when she isn’t really feeling like it. She makes a deal with herself that she has to practice for just 15 minutes, and if by that time she’s still not feeling it, she takes a long break. Apparently, it works for her, and why shouldn’t it work for me?
I'm also a fan of the 15-minute time block just for the purposes of tracking time and as the basis for visualizing time as chunks. As someone with ADHD, I have a kind of time blindness that makes it difficult to think about time in more granular immediate forms. I can get my head around a 15 minute block of time, though. I use it everywhere: in many of my productivity tools and also for Maintaining Momentum on long-backburned projects.