I had half term last week and it was less of a "I should do x hours per day", more of a "By Sunday I want to have completed my homeworks and written more notes for Chemistry and Biology".
I think it's better to say that you'll do 3 lots of mini 20 minute revision sets (20 minutes of writing chemistry notes in the morning, 20 minutes of mymaths homework in the afternoon, 20 minutes of biology exam questions before I go to bed) rather than a whole hour of a subject! At least I feel very daunted when I have an hour of work to do outside of a free.