Can I make withdrawals from my PPF account? What is the schedule?

Yes, you can make one withdrawal per year starting from your seventh year. The first withdrawal can be done after the expiry of 5 full financial years from the end of the year in which your initial subscription was made.
The amount of withdrawal will be limited to 50% of the balance at credit at the end of the fourth year immediately preceding the year in which the amount is to be withdrawn, or the balance at the end of the preceding year, whichever is lower, as per the PPF rulebook. Thereafter, you can make one withdrawal per year. The withdrawal amounts are not repayable.
If you would like to know the rulebook paragraph, here it is:
“Any time after the expiry of five years from the end of the year in which the initial subscription was made , a subscriber may, if he so desires, apply in Form C or as near thereto as possible, together with his pass book to the Accounts Office withdrawing from the balance to his credit, an amount not exceeding fifty per cent of the amount that stood to his credit at the end of the forth year immediately preceding the year of withdrawal or at the end of preceding year, whichever is lower, less the amount of loan, if any, drawn by him under paragraph 10 and which remains to be repaid:
Provided that not more than one withdrawal shall be permissible during any one year.”