Eventually temperatures will become high enough so that the oceans evaporate. At 140 degrees Fahrenheit, water becomes a major constituent of the atmosphere. Much of this water migrates to the stratosphere where it is lost to the vacuum. Eventually, the oceans will evaporate into space. That will be in 5 billion years.
Stars leave the main sequence when they stop burning hydrogen.
The sun, a yellow, G-2 star, will then become a red giant encompassing the orbit of Mercury. Mercury will disappear and Venus will lose its atmosphere and become a burnt out planet. The Earth will suffer the same fate, even though it is outside the red giant's immediate reach. My calculations are somewhat pessimistic and present a worst case scenario that does not include the effects of clouds, but they say a billion years. This model was developed with Ken Caldeira, now at Lawrence Livermore Laboratory.