Aaron Marino of alpha m. is answering the question about how many times per week that you can work a specific body part. Once is the answer.
Many times when starting a fitness program, guys get overzealous. The result is the body doesn't heal. Resistance training tears the muscle fibers down which releases lactic acid. Protein builds the muscle bigger and stronger. Repeating this cycle allows the muscle to grow. But without the rest, the muscle doesn't heal or rebuild. Guys can get crazy and overzealous and don't let the body rest.
You won't lose your gains by resting! By over-training, however, you may end up with muscle atrophy or lack of results. You can exercise 7 days a week as long as you change it up and eat properly (fuel to rebuild). If just starting out a workout plan, check out Alpha's playlist for how to start a diet and fitness program. It's a great to tool!