A lot longer than any of our other energy sources, except maybe the sun, so fusion is a good thing to be able to manage, because hydrogen is so abundant.

However, entropy will ultimately kill the universe. Once the entire universe runs out of available energy (which it can manage without any human help), then we're screwed barring radical new physics that can give us answers like "this is how you create another universe and then travel there."

The good news is that it will take a looooooong time before anybody seriously has to worry about that.