Testing radio after final blows?

I have an Optima mkii that blew a final, found nothing else wrong, but blew the replacement within 10 minutes. Still found nothing else wrong so it was replaced again. I ran it swinging full power into a dummy for long enough to satisfy me, maybe 30-45 minutes of tx in 5 minute increments. I tested by taping the ptt and setting it on my phone playing a video.

I'm thinking about selling it, but I don't want it to blow as soon as they key up. How, and how long should it be tested to claim...

Testing radio after final blows?