This is one of those mantras for being a writer. Write every day. According to the Columbia University Break Writing program, the maximum that a writer can write and concentrate for in a single period is 90 minutes to 2 hours -- so that should be your minimum each day. http://www.columbia.edu/cu/gsas/pages/cstudents/dean/break-writing/break-1.html Other than the possibility of working yourself into burnout, it is good advice.
But if you haven't got two hours a day to write, and many of us don't, the advice is still good. You should write every day, and at least for a certain set time. And if all you have is fifteen minutes, then write for that fifteen minutes. Same time, same place, every day. When I was in graduate school, and single, I had rice for dinner every night. It took fifteen minutes for the rice to cook, and that was my fifteen minutes. I hammered away on my manual typewriter until the scent of burning grain told me that it was time to eat.
A problem, though. Most people take fifteen minutes just to start writing. How do you make the most of your fifteen minutes? My trick, especially when I am snatching bits and pieces of time to fill out a handwritten roughwrite, is to spend the time before I write thinking over what I will cover in my writing. Family doesn't need to be talked to, when I am clearing the table or doing dishes. Especially the teenagers. And while driving? Hey, that's what defensive driving is all about, right? Other people watching out while I think, right?
Seriously, there is a lot I can do while also thinking on a story. I am woman, I multi-task. Maybe not driving, unless it is on an empty highway in the middle of nowhere, and I don't have to be watching for deer, but walking, definitely. And while waiting in the check-out line at the store, stuck behind an extreme couponer who is going to try every coupon in her massive notebook. And at red lights.
I work out a lot of plot problems while sitting at red lights in this town.
Write every day. Think every minute. It's supposed to keep you young.