The final, stable version of MIUI 8 is already out. The update will occur in phases, as right now the 15 Xiaomi phones will get the update.
These devices include:
the Mi 5, Mi Max, Redmi 4s, Mi Note,Redmi 2A, Redmi Note 2, Redmi Note 3, Mi 4 China Unicom version, Redmi 1S, Mi 2/2S, Mi 4i, Redmi Note 3G, and the Redmi Note 4G.
The second phase will give the update to:
the Redmi Pro, Mi 4C, Redmi 3S and 3X, Redmi 2, and Redmi 1S.