how long do you believe humanity will last and why?

Humanity is not really the issue. There are still scattered groups of people living in the fast-diminishing wild areas who have lived pretty much as they have for thousands of years and no doubt will continue to do so for a long time.

The concern is that the world's population has spiked up to almost 20 times it's long term level in just a century or so. This population spike depends totally upon the essentially free energy that has been extracted from the Earth in the form of liquid fossil fuels. As this supply peaks then declines the massive population which exists solely based on an ever expanding supply of free energy will be in deep trouble.

Renewables aren't going to cut it. Renewables can sustain a small population in a state of austerity, but no way are they able to compete with the low EROEI of oil. Fusion power is a myth that is perpetually several decades away.

Global civilization is on the brink of the Seneca cliff, and the only questions are when will we tip over it, how steep will the descent be, how deep will it go, and what will be the size of the remnant surviving population.

If you define humanity by it's global civilization, then we have mere decades left at best. If you define humanity as a species, then the survivors of collapse will have rich pickings from the ruins for many centuries to come.

With so many people on the planet it's hard to believe we can go extinct. Even with big natural disasters there will always be (groups of) people surviving and repopulating the planet. Lot's of people by the way who are so called 'preppers', they prepare for any event that might happen.

At least until I get my mortgage paid off.

Based on current events I would say 100 years. You asked for reasons so here are some of mine: depletion of non renewable natural resources such as drinkable water, overfishing, decrease in biodiversity, modern agricultural practices, wars, mass migrations due to climate change,

does not matter to me. I am dead within 20 years

less than 20 years , agw .

Still at least another 10k years.

Civilization as we know it less than a 100 years.

Why because we still haven’t learned to get along and don’t give a crap about the planet