The timing of cooking the salmon fillets will vary depending on beginning temperature of your fish, how hot your pan is, how thick the fillets are, etc. Undercooking is always better than overcooking as the salmon will continue to cook a little from residual heat after it's removed from the pan and you can always cook a little more, but not a little less. Don’t walk away or start texting or your fish will overcook and dry up from inattention. An instant read thermometer is a good help if you’re just starting to cook fish, but your eyes are your best tool here.