How long would it be until life cannot be sustainable on earth

Well people would say when the Sun expands to a red giant? That's in about 5 billion years.
But in fact it may be earlier, when the galaxy Andromeda collides with the Milky Way in about 3.75 billion years ^^
by then, hopefully we will have progressed to be able to travel to other galaxies and other planets which can sustain life. This may rest on our ability to solve the problem of bringing about nuclear fusion in a consistent manner ^^ who knows xD

If no cosmic catastrophe such ans giant meteor impact or hypernova happen, life will die out because the sun is getting hotter. Long before it swallows Earth (in 7,6 billion years), plant life dies. In about 800-1000 million years. Bacteria and other organisms survive longer, but in 1600-2000 million years they will be extinct too because the oceans will vaporize.

But there are also bacteria kilometres below the surface, living from stones actually. They might survive until Earth is destroyed.

Do you mean if humanity goes extinct, and no other technological species emerges?

In *that* case, it's been estimated that the Earth will heat up due to Solar changes about a billion years from now. However, this is still *highly* speculative, especially considering thermophilic organisms.

In about five billion years, the Sun will expand into a red giant (again, this is only if humanity goes extinct) and destroy all life on Earth. So, between one and five billion years from now. Life isn't going anywhere anytime soon.

The Sun is slowly heating and although it will last another 5 billion years, I have heard that it will be too hot for life on Earth in as little as 50 million years. I cannot currently find a link to support this and have always found that number suspect. Five hundred million years sounds more realistic to me.

If you mean in terms of population then I believe that the planet can support more than 12 times the population here at present as long as we get smart and high tech about land use and resources. Timespan wise a very long time however...