The offer details mention:
- For prepaid customers valid on all plans priced ₹239 & above, unlimited 5G Data will work until the validity of the pack.
- For postpaid customers, it's valid until their next bill generation.
So, as I read it, this Unlimited 5G data will be valid for
One cycle only. Hence, for prepaid users, it makes sense to activate this offer on the day you recharge, so that you get it for a full 28 days or 84 days or 90 days based on what you've recharged.
For Postpaid customers, it makes sense to activate this offer once your billing cycle changes so that you get it for a full 30 or 31 days.