to lose 100 lbs, I would say anywhere to a year or more. thats a lot of weight to lose, and you'll want to do it properly, for it all to stay off.
I calorie count, Im thinking of going on ww b.c I love those snacks and what not too much to give up. lol. exercise is the key to it all I think.
dr. Oz said on Oprah, you eat well,and exercise for 30min a day 5 days a week, and the weight will come off.
hope this has helped you.