How long will it take for me to lose 80 pounds?

Hi im about 5'7 to 5'8 and im a girl and am 15 years old..Im on diet right now and ive been on it for abut 2 weeks now, I weigh about 178 pounds and im trying lose 80 pounds to at least get closer to 90 pounds.How long will it take to lose all that weight?