"Graves and Baechle created a more practical formula to determine when it’s time to increase the amount of resistance. It’s called the 2-for-2 Rule:
If you can successfully complete two or more repetitions in the last set in two consecutive workouts for any given exercise, then the load should be increased.
For example, I perform 4 sets of 8 reps of dumbbell bicep curls. If I can perform 10 reps on my final set of bicep curls for two weeks in a row, then it’s time to increase the weight. Remember, if you are looking to build muscle, you’ll want to target a low number of repetitions – but you should be fully fatigued on your last rep. The 2-for-2 Rule helps identify when fatigue is no longer happening!
If you are new to working out, you may be able to increase resistance by 5% – 10%. If you are more advanced, 2% – 5% may be more appropriate. This usually amounts to 2.5 – 5 pounds for smaller muscle groups and 5 – 10 pounds for larger muscle groups. "
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