The elevator ride down should be less than two minutes, but there may be lines getting in.
There are buses between Dubai Mall and the Burj Khalifa metro station, but from the Burj Khalifa itself to the station it is quicker to walk. Google maps shows a 25min walking route, but I remember by jaywalking, it was more like 10-15min.
The metro train frequency is between 3 and 8 minutes, the ride to the airport takes about 26min.
At the airport the metro station is pretty close to the terminal, you wont need more than 10 minutes to the check-in counters.
So I would plan for at least an hour, 90min to be safe.
Credit:stackoverflow.com‘