The first time I tried generative writing using prompts was in a Tin House workshop in Portland with the writer Nick Flynn. I found the experience revolutionary -- for my book project (I was writing DRAW YOUR WEAPONS at the time), for my writing practice, and for my understanding of what it means to be an artist.

Nick always had us write for 7 minutes. He insisted that 7 was the magic number. He'd tried longer and shorter, but 7 minutes seemed to do the trick.

And he's right. 7 is short enough for you to know you can write through it, even if hard stuff comes up. And it's long enough for you to do some real writing, to get into it and explore.

That said, if there is a different number that works better for you, go for it. You're the boss here. The whole point is to practice trusting yourself and your ideas.