If the internal clock is not there, put clocks in the room. Somewhere the child can see without turning round, a timer on the desk, one on the wall by where they work.

If it is not happening now, it is often not being thought about at all. Not faintly, not somewhere in the background. Not there.

You do not have to agree that it will take longer. Add the extra time anyway and let the week prove you right or wrong. If you finish early, you have a spare twenty minutes, which is not a punishment.

Here is a short way to hold the whole difficulty: the now in front of you is too big and the later is too small. It fits in your head at the moment you need it, which longer explanations do not.