We will end when I say we do.
Strange, because if I look at things, people generally survive longer in countries with more technology. And it's that exact technology that is developing incredibly fast. Technology won't kill us. We won't nuke ourselves to damnation. if anything, we won't be gone before our own sun kills us, though I reckon that chance is pretty low anyway. If it's not for some major cosmic disaster we can't do shit about, then we'll die a long time before the so called Big Freeze happens. So I'm guessing several millions of years is a good number to start with.