How long would it take to lose a hundred pounds?

Think 9-15 months: Safe, sustained wt loss in assoc. W/some activity pgm, usually produces 1-2 lbs/wk wt loss (some start faster, but safe, lifestyle & diet change based pgm. Is usually 1-2/wk). So figure 50-ish weeks. You might pick up pace as lose & can be more active, or have plateaus. Should discuss w/your dr ideally for this big of a planned change. Definitely worth the effort, go for it!

That depends: completely on the rate of loss.