While doing a Weight Lifting Program you do not gain mass during the workout but you do get a nice pump. You do create micro tears in your muscles. You get thousands of them in the muscle you are working out. That is where muscle soreness comes from, don’t worry soreness is a good thing, it means your training is working! When you are healing, your body is metabolizing protein and putting it back into those tears filling them up and therefore making you that little amount bigger and stronger. Over time this amounts to huge gains.
It takes anywhere from 48 hours on the smaller muscle groups, such as calves or biceps, and up to a week in some cases for some larger muscle groups including your back or quads. Keep this in mind while designing or using a workout plan.