Why revision hours are not the whole answer
Revision hours only tell you how long you sat down. They do not tell you whether you chose the right topic, practised the right skill, remembered anything afterwards, or protected enough energy to come back tomorrow.
A useful day of revision usually combines three things: a clear topic target, active work that checks understanding, and recovery time. If one of those is missing, adding more hours may not help.
This is a UK-wide student guide, so qualification language needs caveats. GCSE and A level wording is most directly relevant in England, Wales and Northern Ireland. Students in Scotland may be preparing for National Qualifications such as National 5, Higher or Advanced Higher, so they should also check school, college and SQA guidance for their exact course.