If one does a single workout and measures the size of the muscle both immediately before it and a week afterward, then presumably it would be bigger after than before. But what if the second measurement is merely a day afterward, or an hour? How long does it actually take for the additional muscle bulk resultant from a workout to be visible?
There's no simple concrete answer since each of us react to training stimulus differently. Additionally, genetics, proper nutrition, and recovery will play a part in how long it takes to build noticeable muscle mass. Keep in mind, that if you train naturally without drugs, muscle growth occurs incrementally. That means if you are new to resistance training, gains may seem to appear quickly. However, most of us reach a plateau where muscle gains are harder to achieve. That's why it's more important to find out what works for you, take notes, and stick to a lifelong plan.
During a workout, the muscle fibers are damaged. This damage is what triggers the muscle to repair and grow back larger and stronger. The process of repairing and growing the muscles actually takes several days, so you won't see any noticeable change in size immediately after a workout. In fact, if you worked out really hard, your muscles may even appear smaller after a few days because they're taking some time to heal!
It typically takes about 48 hours for the muscle fibers to start repairing themselves after a workout. And it can take up to 72 hours for new muscle protein synthesis to occur (the process of building new muscle tissue). So if you're working out multiple times per week, you may not see much change in muscle size until after a few weeks or even a month or two of consistent training.
Of course, there are many other factors that affect muscle growth, that could speed up or slow down the process, such as genetics, age, hormones, and diet. But the main thing to remember is that it takes time for muscles to grow, so be patient and consistent with your workouts!