Exactly-- there are more factors, too-- it will depend on how much you have to lose to get to your goal, how much weight you have to lose overall, whether or not you have the genes to be a quick weight loser, how much you exercise, how many flex points you use. You just can't tell. I would look back at how much weight you tend to lose a week, and see if you can find an average amount. If so, figure out how much you have left to reach your 10%, and then divide that by the average, and that would be how long it will likely be to reach the 10%.
of course, there are other elements-- your body may plateau before then, and that's an extra week. The important thing is to realize that you are healthy, and working towards the goal, and just to be happy where you are!