My freewrites are often inconsistent in length, haha. If I'm lucky I get to write longer within the time limit, but in all honesty if the story isn't done most of the time I'll just let it flow and continue writing. The whole purpose of the freewrite exercise is that we continue writing and let our imagination speak, so the time is just a guide. I usually try to come up with the idea of the story first before I start writing, that way I get to write faster :P