A post on the Xiaomi’s official MiUi forum has listed information about the MiUi 6. The MiUi 6 based on android KitKat and Lollipop (not confirmed) for Redmi phones timeline reveals the date when the specified Redmi models will get the update.
Unfortunately at the end of the post it is mentioned that the ROM’s refer to China ROM and not Global ROM. This means there is still no word as to when the Indian version will get the update. As per the post MiUi 6 based on Android 4.4 have already been rolled for Redmi 1S 4G version.
Redmi Note 3G will get MiUi (android 4.4) starting from 13th January 2015, Redmi 1S 3G from Feb 2015, Redmi 1 from Mid Feb 2015 and the Redmi 1S 3G will start getting MiUi 6 based on Lollipop (not confirmed) from end of March 2015.
Other Mi Phones like the Mi 2A, 2S, Mi2 will start getting updates (Android L) in January 2015 . For more info check their official forum.