To ensure that your HDFC Bank credit card is not closed or converted from lifetime free to chargeable, it's recommended to use the card at least once a year. Normally, credit card providers won't close the account due to inactivity, but according to HDFC Bank's terms and conditions, if the card isn't used for 15 months, they may convert it to a chargeable card or close it. To avoid this, it's advisable to spend a small amount (around 10-15 Rs) on the card, just to be on the safer side.
It's important to note that not using the card won't affect your CIBIL score, as long as you are regularly and properly paying on your other credit cards.
Lastly, there is no specific timeline for using the card. Simply use it when you need to and make the payments on time.