Code reviews are a reactive exercise. They allow people of influence to criticise work. But it is important that people of influence have first stepped up, and laid out a clear direction.
As the mentor, you should have a mental plan for their one month, three months and eighteen month progressions. If you do not have this in mind, you should not have hired them.
From here, you should set a tempo that steers the junior to grow. The things you are growing are their skills, judgement and initiative. You need to balance the need to make sure they are heading down a good path against the momentum of their own initiative, which is valuable but also prone to misfiring.
This path will be different for each person. So: have a plan, but be prepared to adjust it regularly.
There comes a crossover point where their judgement and ability becomes strong. You realise that your steering efforts are holding them back vs their own initiative. There should be some conversation where you explain that you are stepping back, and that they should be careful with the extra rope they will be getting. With graduate hires this will be at least eighteen months out.