Originally Posted by aiptasia
If you don't know your starting point, you'll never know exactly when it's done. It'll be a guestimation on your part. To answer your question on fermentation, yep. It's definitely possible to have a fast ferment with warm temperatures.
Of course you can know when it's done. There's only a finite amount of sugar in there, and I'm pretty sure fermentation never lasts longer than 2 weeks or so, and usually less than that. People let it sit in the carboy to age anyway so it'll be fine.
Plus you can always see if the sg stabilizes over several days.