I wonder what you define as heavy usage for AirPods. I have 2 pairs, one of which is the first generation and I use only for running. I run almost every day for around an hour. I've had them for around 2 years, probably a bit longer. They still last more than 3 hours per charge. Maybe even 4. Are you using them for 4+ hours a day? Also, is there a way to interrogate them to see how many hours of usage they have or what the remaining battery life is?

Edit: I've had a much tougher time with the Apple Watch I pair with the AirPods while I run. My first one only lasted about 1.5 years before its battery became unusable for running.