That surely seems like an awful lot of weight in a short amount of time. Honestly, just focus on your lifestyle and diet changes, and as the weight starts to come off you'll start to feel amazing and will probably forget all about having a particular goal! If you make sound healthy decisions and you aren't starving yourself, your body will dictate what a "safe" amount of weight is to lose in that time frame. I think that expecting to lose somewhere around 40 pounds (10 pounds a month) seems most realistic, but again, that's for your body to decide.
Getting healthy should be a reward, not a punishment, so don't make yourself miserable, and give your body all nutrients, rest, love, and patience it needs!