The UKBA have introduced a 5 year limit to study carried out on a student visa, at undergraduate and Masters level. Some exemptions will apply for students studying undergraduate courses which are recognised as being longer than 3 years.
If you have studied at undergraduate or Masters level in the UK, the University will need to consider this to decide if a CAS can be issued. As part of your application process you will be asked questions about previous study; you must make sure you answer these questions correctly.
Some students may also need to carefully plan their studies to make sure they have enough time to complete their courses, for example if you are internally repeating a year of a course.
For further information please see the UKCISA student visa page.