I recently availed IPL DDD 50-Day Free Offer on my existing JioFiber prepaid connection by recharging for the 300 Mbps annual plan.
This is how it works:
First, when we recharge for the annual plan, it gives us 390 days of validity which is 360 days + 30 days. Then after some days, a voucher called "IPL Offer voucher" of 20 days for the same plan is added to our account. We get SMS and email alerts on our registered contact details as soon as it is added.
As per Jio, the voucher will be valid till the next 2 years from the date of allocation. Unredeemed vouchers will expire post 2 years since the day given.