I do this type of exercise in the morning but get prompts from CNN or a favorite news channel. Once I choose the prompt, I mute the sound and write for as much as 15 minutes, more usually 10. I know, I have bent the idea of free a bit, but, works for me.
I use freewriting in my speaking and listening classes as a five minute warm up exercise. It sounds counterintuitive, after all, it is not a writing class. My students of English as a foreign language respond to a prompt, usually one word but maximum four. They do this twice a week; although I collect and use them as a source for topics of interest, I do not evaluate them in terms of grammar or organization. That would destroy the idea of “free.”