Prepared salad lasts about 1-5 days in the fridge, depending on the freshness of the ingredients beforehand.
It also depends on what the salad is made up of.
We will assume that your salad is traditional and is made up of lettuce and vegetable produce.
A salad of this sort will last a few days (2-3) before it loses its crisp freshness, but again, it also depends on how fresh your lettuce and other salad components were, to begin with.