I believe that this is something writers often ponder. I've read interviews of prominent authors, attended panel discussions, participated in writing groups, taken writing courses, etc. and usually at some point in the discussion, this question will pop up like a shiny, red pimple on the morning of the prom. The truth is, various writers will give various answers. Some well-established storytellers will tell you to write everyday for hours at a time. Clearly we don't all have this luxury--I sure don't, but I'm working on it! Here's my personal opinion on this widely debated writer's conundrum.
On average, I write five to eight hours a week. When I say "write," generally what I am referring to is my novel-in-progress or one of my many, many underdeveloped short stories. For the most part, I don't include writing this blog or engaging in other forms of social networking as part of my writing time. I am solely commenting on my 'work,' or in other words, the pieces I hope-upon-hope, wish-upon-wish will one day be in print.
I teach both day and evening courses, and as a result, certain days of the week are unavailable for writing. The benefit of this is that on other days, I have a light course load, which leaves plenty of (non-excusable) hours to enter the thriving world of my own creation. In other words, it is necessary that I both find and make time to be the scribe I so desire to be. On Tuesdays, I'll grab an hour after work. Thursdays I finish teaching at 3 pm, hence I can fit in two hours or more. This quarter, I am off on Fridays, so I take at least a three hour chunk to devote to the art. Then there are the weekends, which depending on my level of 'open availability,' I either have two extra Fridays, or, sadly, a Monday or Wednesday (days in which writing is not possible).
Of course when the quarter changes I'll have to formulate a new plan, but until then this is what I work with. Are there some weeks when I write only three hours? Of course; life's ebb is ever changing, but the key truly is persistence. If I miss a day, I'll find another where I can make up for it. But regardless of the unpredictability of time, I've been working with this (albeit inconsistent) schedule since August 2010. The truth is, though I am still in the midst of perfecting it all, I really have made considerable progress.
I think the key is this: Look for time, make time, and utilize time. You know your schedule better than anyone else. But if you decide to write, you need to sit down and do it. Trust me, in this enterprise, it truly is the only way.