For me, the ideal sprint remains the one Ramona taught me: one hour, first thing in the morning, before the day takes over, with a goal of writing 1000 new words. This is the gold standard, the one to which I aspire.
The reality, however, is somewhat different.
On a good day, my one hour sprint consists of sixty uninterrupted minutes sometime early in the day. Because I have a high school student living in my house, and because procrastination is part of my writing process anyway, these sixty minutes rarely occur right out of the gate. I usually putter for a bit, checking email and often writing and posting my blog before settling in for my sprint, which occurs anywhere between mid-morning and bedtime.
Perhaps the most useful sprint variation I've stumbled across is the sprint as a means of digging into an unpalatable or overwhelming task. For me, that task is revising. I hate revising, but there's something about breaking revisions into one hour chunks that makes the process much less painful. By telling myself I have to revise for only an hour, I've already taken an insurmountable task (revising an entire novel!) and made it more approachable.
I rarely "cheat" and count writing I'd do anyway (blog posts, for example) as my sprint time. Regardless of how I do it, when I do it, or how many chunks I break it into, I think sprinting is meant to advance a work-in-progress.
What task do you want to tackle, but keep putting off? Could a sprint be the answer for you?