Zehra Cranmer asked if I'd do a blog post about it. So here it is, Zehra!
Freewriting is simply writing without pausing. You can type. You can write longhand. Not pausing is the only rule.
So how do you know when to stop? That's up to you.
Some people set a timer. This is a good strategy if you have a difficult time settling in, stilling the outside mind-chatter, and writing. Figure out what amount of time you need to spend before you stop jumping up or feeling the need to click over to another screen. Sometimes it only takes five minutes of constant writing to find your zone.
I also use the timer if I'm worried about something and that thing keeps intruding on my writing time. I'll say to myself, "Self. . . you have ten minutes to obsess on this. Go." Then I dump it all. After ten minutes, I'm done. It's time to move on to something else.
Setting a timer also works if I'm trying to answer a plotting question. In this situation it becomes more like brainstorming. Instead of letting my mind wander where it will, I tell myself to answer something specific. I might freewrite for ten minutes, trying to find a name for my character, running through all that name's associations to different names or settings or circumstances. This is a great strategy if I'm avoiding a problem in my manuscript. Or if there's something that's bugging me about a scene, but I can't quite put my finger on it. Instead of looking sideways at the issue, I make myself focus for ten minutes and I'm done.
Another option---and the one I've been doing lately---is to write until you reach a certain number of words. I've been doing this because the first thousand words of the day are always the most difficult for me to write. After 1000 words, solid bricks of words become liquid. One thought flows into the next. Usually I write those words into a novel or a story and then I have to go back and revise like crazy. I don't know why it took me so long to realize I need to warm up with disposable words.
Well, yes. Maybe I do know. It's the idea of it. The idea of throwing away 1000 words every day makes my heart race a little.
The idea became a lot more palatable when I timed myself. I can easily freewrite 1000 words in 15-20 minutes. At the end of that time, my thoughts flow.
When I attempt to dive straight into a novel or story, it usually takes me an hour to write 350 words. In the second hour I write 500 words. In the third hour I take off. Why? Because I don't cross 1000 words until that third hour of novel writing. That's how many words it takes before I find fluidity. To make matters worse, those first thousand words need more revising than the rest.
If I do my freewriting warm-up, I'm in the flow of the novel or story from the first words I put down.
If I spend 20 minutes burning off first words, I write better and more in the time remaining.
Thanks for the question, Zehra! Do remember that none of this is set in stone. Adjust and modify the process into something that works for you.