As a natural lifter, your body can only gain so much muscle, even if you're doing everything else right.
If you're a beginner, you can enjoy some beginner gains. Most beginners can gain up to 30 lbs in one year provided you focus on your nutrition, most of that muscle will come within the first 3-4 months.
After you've gained your first 20-30 lbs of muscle, you're an intermediate. Now, muscle growth will slow down. Expect to gain 10-15 lbs maximum over the course of the next year.
After 2 years , your muscle growth will almost come to a halt. After all, you've already gained 30-45 lbs of muscle! After that, expect to gain between 2-7 lbs of muscle per year. At this point in your life though, you'll probably be VERY HAPPY with your physique, so replace the slow gains with other goals like getting leaner, stronger, or focusing on a lagging body part.