There are all sorts of nasty things that we know will happen eventually: e.g. the sun turning into a red giant, puffing up and engulfing Earth; or our galaxy crashing into Andromeda (nearest other galaxy) and being torn apart! All of these are billions of years away though.
The sad reality is that we humans are most likely to cause our own destruction, either through war or climate change. If people don’t start to take responsibility for global warming, then we may only be talking thousands of years until our planet is uninhabitable…
In other words, civilised humanity might already be half way through its time on this planet!
I agree with James’s answer. To add a few thoughts, I defnitely agree that we are headed toward disaster, and if we continue down the trajectory of war and environmental adversity, world will end. Might take anywhere from a month (nuclear war) to several thousand years.
But then as a race I also have faith that we will stop ourselves short and make policies which will avoid a global disaster from happening.