It is true that monthly investments is the best way to go for PPF accounts as then you get interest regularly. It is also advisable to do so to prevent frivolous spending of hard-earned money!
This advice was given to me too and when I was having a full-time job I did put each & every month a small amount without fail. However, once I started part-time work, I found I was not necessarily able to put aside money each month to put in the PPF account. As of today, I put in once or twice a year a lump sum amount after my earnings have accumulated in my bank's savings account. The best time to put such a lump sum is as soon as the financial year ends on March 31st. That is, you should put in the amount between April 1st and April 5th. That way, the amount accumulates interest for the entire forthcoming financial year. If you do have some lump sum later too, then put it before September 30th as this is when the banks have their half-yearly closing day.
Thus, basically for putting a lump sum the best time is as close to the beginning of the financial year as possible. Furthermore, a C.A. told me that the best time to put in any money in a bank account (whether PPF or your salary or any other income) is before the 5th of the month (from 1st to 4th) to get interest.
Keep in mind that once your PPF account matures, you can extend it for a further period of 5 years maximum as informed to me by my Bank.
When people come at you with their worst, you should come at them with your best (advice given to Selena Gomez by her mother, quoted in Time magazine.)