How long should it take for you to lose about forty pounds?

to lose it in the most healthy way and to make sure you don't instantly gain it back, it will take 6-12 months.

Reason for this is the body needs to adjust to your new diet (diet being the new way you eat, don't go on a diet for little bit, and then once you lost the weight quit it, you WILL immediately gain you weight back if you don't maintain that healthy diet)