How long should I cook my corned beef?

I cooked a 5 pound corned beef in very gently boiling water for 2 hours, turning every every half hour. (I am at 100 m above sea level if you are a purist.)
After coming out of the boiling water, remove the surface fat and top with a glaze: I cup apricot preserves, 2 TBS of soy sauce and, ¼ cup of brown sugar.
Then put it in the oven at 325F for 40 minutes.
Let it stand 15minutes before slicing.
I drizzled the juices and glaze on the sliced pieces. It was tender and delicious!

Instructions are vague—-“Cook 3 to 5 hours.” I would simply simmer (if you are doing it on top of the stove) for an extra 30 minutes and take a little taste.

Personally, if I were doing the traditional corned beef with cabbage, carrots and boiled potatoes, I would skip the glaze and just put a little butter on the vegetables.

I cooked a 2.5-lb. one in my slow cooker last Sunday. Took 8 hours and came out wonderfully tender. I used to use a pressure cooker, which took far less time, but didn’t like the corned beef smell that permeated my hosue for days.

I did a 5-lb one last week for 3 hours very slow simmer in a cast-iron casserole on the largest burner. Quartered cabbage for the last ten minutes. Inch-long pieces of carrot, quartered potatoes and halved smallish onions for the last thirty minutes. There was pickling spice already in the brisket packaging or I’d have added some. It was melty and salty and beautiful. No butter; indigenous sauce.

Someone told me that they used a crock pot and let it go 12 hours. . It was very tender. Almost too tender to slice without falling apart. They had to cool it outdoors (or in the refrigerator) so it would stay together enough to slice.
That seems like a good data point for the other extreme.

A 3–4 hour slow covered cook on stove top works for me all the time.