You don't grow during your workouts. You grow during your time off of the gym. That is when your body repairs the damage of your workout and you grow.
That being said, you should always be giving your body protein so it has the necessary ingredients to make you grow and repair. That's why you need to eat a balanced diet, yadda yadda yadda.
Summary: Shoot for a minimum of 1 gram of protein per pound of mass you have. Spread it out eventually through 5-6 meals EVERYDAY.