Of course, if it's warm enough in your room, it doesn't matter.
Another way to cook lamb's liver tenderly is to never leave the pot on very low heat, keeping it seemingly on, seemingly off.
It's not just sheep's liver, all animal livers can be done this way.
Sheep liver specific simmering bubble how long to cook is not good to say, because the size of the different, each person with the fire is not quite the same, including tools, including the amount of water in your pot, let's say in summer, we pot of water boiling, it cools down slower, this time may be cooked faster, if it is the case of the winter, especially in the north of the winter, if you do not have any thermal insulation kitchen If it is winter, especially in the northern winter, if you do not have any insulation in the kitchen, in fact, the cooling speed is very fast, so this time it is recommended that you add more water to the pot, so that its temperature down slowly, easy to add less, it is likely that the water is not cooked on the cool. General forty to fifty minutes should be about the same. You can use a bamboo skewer to penetrate, as long as it is not out of blood, that is basically almost.
If you cook with high heat, sheep liver is certainly not tender, because the high fire then there is a problem of osmotic pressure. It will squeeze the water out of the sheep liver, so he becomes old and hard, if you want it to be tender, either the water that I said seems to be on or off, and keep cooking it, about 35 minutes almost, I estimate.
Either cook it for a minute or two and simmer it in a little more water. It definitely came out tender.