Only xiaomi knows. Should be soon if everything goes well. I hope.
But you will see almost any difference between nougat and Oreo in miui daily usage for the user interface. Miui 9 runs its mask with all its functions.
However Its nice to see updates, system wise and security patch's. Something miui seems to neglect a little sometimes and takes forever. And the phones base increasing month after month it won't get better, sadly.