There are no legal requirements for how long you have to return to work after maternity leave before leaving your position; however, you should give your employer your contractual notice period. If you wish to return to work earlier than stated, you must give your employer at least 8 weeks’ notice of your new return-to-work date.
Although there are no legal minimum terms, your employment contract will outline how long you have to return to work following maternity leave to prevent any contractual pay having to be paid back, so be sure to check this before giving your notice.