Completing your studies early will likely lead to a reduction in the amount of time you will spend in the UK on a Student visa. This is because the university are obligated to report your early completion to UK Visas and Immigration (UKVI). After this, you will receive an earlier visa end date from UKVI. It isn't possible for the university to predict exactly when your visa will be cancelled and what the new expiry date will be. This is only revealed when UKVI take the cancellation action and write to you with these details. UKVI do not share this information with the university.
The Home Office advises if your visa is cancelled following early completion of studies and you travel abroad, or you are already outside the UK at the point of cancellation action, your Student visa will lapse and you will not be able to re-enter the UK on the same visa. See Article 13(3) of the Immigration (Leave to Enter and Remain) Order 2000.