The transition to 5G from 4G could happen much faster compared to the switch from 3G to 4G services.
Indian government has NOT allocated airwaves for 5G once 5G airwaves(spectrum) is allocated then the race will begin between Jio, Airtel, and Vodaphone-idea.
The government recently said it plans to allocate airwaves to offer 5G services – which can support download speeds 50 to 60 times faster than 4G – by end of 2019.
If sources are to be believed then JIO can have a fully operational 5G network within 6 months of airwaves allocation. so India should get 5G by 2020.
Considering the scale of the project you can add 6 to 12 months, as these projects are prone to delays. so by 2021, we should definitely have 5G. (figures crossed)
if you want to know more about 5G and how it works: http://learnersglobe.com/what-is-5g-%F0%9F%8C%8E%E2%9A%A1-%E2%9D%93%E2%9D%93/