I've done some freewrites. In fact, I've gotten some nice upvotes on some of them. I love doing freewrites, but nowadays I've been using my own prompts and my own timings.
But in Marianne's, I used to do exactly 5 minutes because I felt like the purpose was to exercise restraint. Everybody has their own purpose when they do them, I guess. Some are trying to escape the writer's block, others just like community stuff :)
I'm writing quick right now because I'm about to make another story. I leave you to go write that one!
I do the freewrites almost every night although I'm also currently in the middle of two fiction series. I tend to write chapter based stories that are about 5 to 10 chapters long. There is one that I'm working on that is probably going to be about 50. I look forward to reading your story.