Running?

There is no such thing as spot reduction. Yes, it will slim your thighs if you are in a caloric deficit but how long it will take depends on a lot of factors such as: how big your caloric deficit is, where you gather fat, and where you tend to lose or hang on to fat when you lose weight.

So basically, there is no simple answer but if you lose 5 lbs you should notice a reduction in the size of your thighs but the exact lose will depend on genetic factors and how long it takes to lose 5lbs depends on your diet and exercise.