When will the Sun stop producing enough light and heat to keep the Earth habitable?

Well..once it flares up into a red giant in approximately 6 billion years or so, it will eventually fade away into a white dwarf star with a luminosity less than 1 percent of its current power output. This will happen by about 7 billion years from now, give or take a few millenia! Once the power output of the Sun dips below about 50 percent of its current brightness, the surface temperature of the Earth will never rise above the melting point of water, so there will never be a protracted era when we could comfortably survive on Earth after the red giant phase ends. Surving the red giant phase itself will be a major challenge since the surface of the Sun will be about where the orbit of Venus is now. But with the inevitable green house heating, the oceans will boil and evaporate in about 700 million years or so, so we are getting pretty close to the end of a habitable Earth already!