It will keep beeping until you either take the food out or at least open and then shut the door.
And if you use the express cooking buttons (usually 1 through 6) you can stop it whenever you want just by hitting stop or opening the door. Then hit start again if you need more time.
My microwave in the TT is only about 900 watts and the one in the house is 1100; so sometime you have to adjust the cooking times depending on the wattage of the microwave. Longer if its a low wattage and maybe a little less if it is a high wattage. Just need to get used to it. And read the book that comes with it.