Unfortunately, that is not the answer to my question. I understand how far/high it can fly from the controller. What I need to know is how well it performs at an elevation of 7,000 above sea level where the air is considerably thinner than at sea level.
In that case, you will get less flight time, since the motors will have to work harder to move the amount of air it needs to stay in flight. How much shorter per 1,000 feet, I don't know. I have not made these tests. Highest I've flown is 400 feet above sea level.
I am guessing that you live in a high elevation area. Sadly where I live, the highest mountain is around 4,000 feet and it's off limits to the public. If not I would have loved to go there and do testing for you. If you already own a Tello, your best bet is to take off and let it hover until it uses the battery and lands. Take the time and we can do the math to calculate how much time you lose with altitude. I can confirm that the battery lasts 12 minutes 58 seconds of flight time at sea level.
I normally fly at sea level but recently I flew in the mountains around Lake Tahoe (USA) and only noticed one change in the Tellos flight characteristics - it was a little slower to respond to the altitude (up & down) controls. I assumed it was due to thinner air. Just a little slower response at 2100m (about 6,900 ft) than at sea level. All other controls left, right and yaw responded the same as at sea level. I noticed no shortened flight times on my flights at altitude. I did not think to perform any measured tests. This is all based on my past experience. ( I was too busy taking videos and pictures of the scenery around Lake Tahoe).