Don't know how long it'll take, xiaomi is struggling even to provide stable nougat to its phones of course it is miui with huge customization so time taking is normal for it.
But why in this case of stock Android tv it is taking so much time for the updates,
Maybe forever. They were careful to say that proper HDR support would merely be possible with an upgrade to Android 7.0.
They said they planned to release Android 7 Q1 of 2017, but their priorities might have changed.
It's almost impossible to sue a Chinese company from outside of China and collect damages. They may never deliver. They will go on selling the box at Walmart and suffer no consequences. It's the risk you take when you buy from a lot of Chinese or Taiwanese tech companies.
[UPDATE] This morning, TV decided that HDMI 1 input is HDCP 2.2. I really do not know anything about anything between TV, Mi Box, HDMI cable, software ...
Between 1 second and 10,000 years.
Not sure what answer you expect to get from that question.