CAS are only valid for six months and can only be used from three months before the start of your programme. If you accept an unconditional offer more than three months before the start of your programme then you will likely receive an email detailing your CAS information around three months before the start of the programme. If you accept an unconditional offer within three months of the start of your programme, we will issue you a CAS within three working days of your unconditional offer being accepted - https://www.qub.ac.uk/new-students/postgraduate/new-international-postgraduate-students/immigration-visa-cas-procedures/