Top right corner in the app, three little dots:
"..."
"Schedule power on/off"
It's not really a timer (turn off in xx hours/mins) but a schedule mode (turn on at 6pm, turn of at 12am).
Then if you want even more control you can use "Location mode" (turn on every day/weekday/etc.) at 7pm, turn on when you're at home (uses Wifi to detect your presence), turn off when you leave, turn on when the windows are closed (buy the Xiaomi smart home sensors), etc.
Can also manually set the sensitivity (turn on when AQI is >50, etc.) but not very useful locally because it's calibrated for China so the lowest threshold is 50, which we don't get unless it's very hazy.
Is yours chinese mi home app?
I cant find it from mi home app in google store. Mind share the app?

