I'm 5'6 and weigh 170. I want to start over...how long do you think it will take me to lose 50 pounds?

Im moving in with my boyfriend(iv recently GAINED all of this weight 45 pounds) i would like to get down to 120 with a strict regimen. If i do so...roughly how long do you think it will take? ANY PERSONAL EXPERIENCES GREATLY APPRECIATED!! Ill give the most points possible, i would just really like to get an estimate date to look foward to to...because I believe it would motivate me!! thanks! :)