Remember – you cannot work full time before the official programme end date that was stated on your CAS. When you are granted student immigration permission you are given an extra number of months after the end of your programme to obtain your results and graduate. The amount you are granted depends on the length of your programme:
Undergraduate students: you are granted the length of your programme plus four months
General Course students: you are granted the length of your programme plus two months
Master programmes less than 12 months: you are granted the length of your programme plus two months
Master programmes of 12 months or longer: you are granted the length of your programme plus four months.
UKVI considers the extra months after your official programme end date stated on your CAS as vacation, so you will be able to work full time. However, you are still restricted by the Tier 4 conditions so you are unable to take a permanent (open-ended) full time contract until you are in a position to switch into visa route that allows permanent, full time work .
Tip: Don't forget that the Home Office have a record of your official programme end date as this was provided on your CAS.
If your employer offers you a permanent contract before you can apply to switch visa category, you may need to request that they hire you on a temporary contract until you can apply for your new visa.