If you don't care for the taste, you have two options.
Optimized for comfort
Start buying uniform size fish. Cook it through using a guess for the time as a starting point. You just pull the fish out, try eating it, and if it is raw in the middle, you put it back into the pot. Record the time the fish took until it was cooked through. The next time, repeat, but leave it inside for the duration it took the last time before checking the first time. Repeat as needed, until you pull out a fish that was through the first time.
Now slap a 25% to 30% security factor - e.g. if you got a cooked fish after 30 minutes during the tests, define your new "cookign time" as 40 minutes. From here on, every time you cook fish of the same size (which is easy if you are buying those shaped frozen pieces), cook with that timer and turn off when done. For even more convenience, use an appliance that can turn itself off by timer.
Optimized for speed
Buy a thermometer. There are tons of different kinds, and some of them can be submerged. You can even buy one that has a sound alarm when a temperature has been reached. Make sure you always stick the thermometer deep into the middle. Use a table for doneness to decide how much you want your salmon cooked. Then boil by temperature.