Your DD2 sounds really sensible, ww.
I think it is a bit tricky for students because the examinations still seem a long way off. They probably haven't finished a lot of their courses. Then they will have study leave from at some point before half-term.
However, it seems like the examination season is getting earlier and earlier. Our pupils are doing quite a few before they even start study leave.
Good advice is that they should be working hard over the holidays. There is a lot of productive work they can do to make sure that they give themselves the best chances of doing well in June.
I would be very sceptical about A grades for mocks, btw. They can't really get an A when they are only 75% of the way through a 2-year course (perhaps luck in one subject but not A/A* across the board).
At our school, when we give mock grades, we give them on the basis of doing the real thing tomorrow, not on a prediction of how they could do if they all their future work falls into place and they work to their best.
Last year, my DS1 did his GCSEs - he got 8 A* and 2 A grades, and an A grade in AS French. He probably did about 3 hours per day of revision during the Easter holidays.