It will download the updates when it is on, there is no need for it to be on all the time, you just leave it on for long enough to download updates. (usually Tue|Wed).
I have mine set to inform me of updates, but to download and update only when I choose.
I have learnt through bitter experience that, if left on fully automatic, the computer can re-start at some very inconvenient times, for example when I am doing something important online.
MS have configured it to detect when you need to shut down and leave in a hurry so will always only update laptops at that time, and it will take it's longest when you next need to boot for an important meeting.
MS have configured it to detect when you need to shut down and leave in a hurry so will always only update laptops at that time, and it will take it's longest when you next need to boot for an important meeting.