My 560D manual says at least 30 seconds when above freezing, and at least 90 seconds below freezing. It says to keep the button depressed while starting and until the smoke clears.
Mine will start without them on a hot summer day, and about 20-30 seconds when around 40 degrees. If you look at your watch while doing this you will discover 90 seconds is a long, long time. I had trouble starting last winter until I read to keep it depressed even after it caught - that made all the difference.