How long should I try a diet/diet change to measure its Effectiveness? Is two weeks long enough or a month or two months?

Hey Marc, 

I am 20 years old, 5'10 inches and 140ish pounds. I have gained a lot of strength over the past few months, but I do have some lower ab and lower back fat. I was wondering how long it would take for a change in diet to show conclusive results?

Thanks