A vice president can act as a president maximum for the period of:

As per  Art 65 of the Constitution, the Vice-President acts as President, during casual vacancy in the office of the President by reason of death, resignation or removal or otherwise, until a new President is elected as soon as practicable and, in no case, later than six months from the date of occurrence of the vacancy . When the President is unable to discharge his functions owing to absence, illness or any other cause, the Vice-President discharges those functions till the President resumes office. During this period, the Vice-President has all the powers, immunities and privileges of the President and receives emoluments and allowances payable to the President.