Look, I know you want a number. Fine. Most mussels take 5 to 7 minutes over medium-high heat, but that depends on your pot size and how many you’re cooking. The real trick is to listen.
When you first add the mussels, you’ll hear a gentle hissing and clattering. After a few minutes, that sound turns into a soft, rhythmic steam. That’s when you start watching closely.
As soon as you see the first shell pop open, give it one more minute. Then kill the heat. The residual steam will finish cooking the stubborn ones without turning them into little erasers.
