If the starting temperature is 20C then to get to boiling you need to add 80 kelvin * 1000 gram * 4.2 joule/gram = 336 kJ to get to 100C. At 1000 W that takes 336 seconds, or 5 minutes 36 seconds. In practice my 850 W microwave takes nine minutes to raise a litre of milk to 85C from fridge temperature so I don't think there is too much risk of boiling.
