There is no fixed deadline for accepting or declining an offer of place. However, students applying for general bachelors' degrees should make a decision as soon as possible because you cannot proceed to course enrolment until you have accepted the offer. Deadlines often apply to other phases in your enrolment, such as selecting papers and submitting a declaration.
Offers for places in specialised programmes may require that applicants make a decision by a specific date, after which any outstanding offers may lapse.
If you accept an offer you can still cancel your application later if necessary.