I think it just sort of depends on how much effort you want to put in combined with what your body will let you lose (that part isn't up to you, unfortunately!!). I've lost all of my weight since right before Christmas 2010. But I've been strict with my diet and exercise routine--60 min 5-6 times a week on the elliptical and carb controlled 1500 calorie diet. Controlling the carbs seems to be a big trick for me.
Good luck! Even if you aren't THERE by your bday, at least you'll be getting much closer!!