20 months is roughly 4 months. If your diet and training is on point, you should be able to put on a solid 4-6 pounds of muscle mass in that time.
Based on your question it sounds like you are new to working out.
Here is a really good video for you to watch where I explain how to workout as a beginner, intermediate and advanced lifter ->
http://www.evolutionary.org/forums/...inner-intermediate-advanced-lifter-47320.html
I see a lot of new lifters following a 5 day split workout routine. That will work but in my opinion, you can make muscle and strength gains much faster by doing full body workouts 3 days per week. Being new to lifting, you don't need a huge amount of volume to stimulate a muscle to grow. By doing full body workouts 3 days per week, you are stimulating a muscle to grow 3 times per week verse just once per week in a split routine.
I would do a workout like this MWF -
Squat 5 x 5
Deadlift 5 x 5
Bench Press 5 x 5
Wide grip Pull up 5 X 5
Bent over Barbell row 5 x 5
Overhead Barbell shoulder press 5 x 5